Fix hstore_plpython for Python 3.

In commit d51924be8, I overlooked the need to provide linkage for
PLyUnicode_FromStringAndSize, because that's only used (and indeed
only exists) in Python 3 builds.

In light of the need to #if this item, rearrange the ordering of
the code related to each function pointer, so as not to need more
#if's than absolutely necessary.

Per buildfarm.

contrib/hstore_plpython/hstore_plpython.c

@@ -11,20 +11,22 @@ extern void _PG_init(void);
 
 /* Linkage to functions in plpython module */
 typedef char *(*PLyObject_AsString_t) (PyObject *plrv);
-
 static PLyObject_AsString_t PLyObject_AsString_p;
+#if PY_MAJOR_VERSION >= 3
+typedef PyObject *(*PLyUnicode_FromStringAndSize_t) (const char *s, Py_ssize_t size);
+static PLyUnicode_FromStringAndSize_t PLyUnicode_FromStringAndSize_p;
+#endif
 
 /* Linkage to functions in hstore module */
 typedef HStore *(*hstoreUpgrade_t) (Datum orig);
-typedef int (*hstoreUniquePairs_t) (Pairs *a, int32 l, int32 *buflen);
-typedef HStore *(*hstorePairs_t) (Pairs *pairs, int32 pcount, int32 buflen);
-typedef size_t (*hstoreCheckKeyLen_t) (size_t len);
-typedef size_t (*hstoreCheckValLen_t) (size_t len);
-
 static hstoreUpgrade_t hstoreUpgrade_p;
+typedef int (*hstoreUniquePairs_t) (Pairs *a, int32 l, int32 *buflen);
 static hstoreUniquePairs_t hstoreUniquePairs_p;
+typedef HStore *(*hstorePairs_t) (Pairs *pairs, int32 pcount, int32 buflen);
 static hstorePairs_t hstorePairs_p;
+typedef size_t (*hstoreCheckKeyLen_t) (size_t len);
 static hstoreCheckKeyLen_t hstoreCheckKeyLen_p;
+typedef size_t (*hstoreCheckValLen_t) (size_t len);
 static hstoreCheckValLen_t hstoreCheckValLen_p;
 
 
@@ -34,29 +36,34 @@ static hstoreCheckValLen_t hstoreCheckValLen_p;
 void
 _PG_init(void)
 {
-	/* These asserts verify that typedefs above match original declarations */
+	/* Asserts verify that typedefs above match original declarations */
 	AssertVariableIsOfType(&PLyObject_AsString, PLyObject_AsString_t);
-	AssertVariableIsOfType(&hstoreUpgrade, hstoreUpgrade_t);
-	AssertVariableIsOfType(&hstoreUniquePairs, hstoreUniquePairs_t);
-	AssertVariableIsOfType(&hstorePairs, hstorePairs_t);
-	AssertVariableIsOfType(&hstoreCheckKeyLen, hstoreCheckKeyLen_t);
-	AssertVariableIsOfType(&hstoreCheckValLen, hstoreCheckValLen_t);
-
 	PLyObject_AsString_p = (PLyObject_AsString_t)
 		load_external_function("$libdir/" PLPYTHON_LIBNAME, "PLyObject_AsString",
 							   true, NULL);
+#if PY_MAJOR_VERSION >= 3
+	AssertVariableIsOfType(&PLyUnicode_FromStringAndSize, PLyUnicode_FromStringAndSize_t);
+	PLyUnicode_FromStringAndSize_p = (PLyUnicode_FromStringAndSize_t)
+		load_external_function("$libdir/" PLPYTHON_LIBNAME, "PLyUnicode_FromStringAndSize",
+							   true, NULL);
+#endif
+	AssertVariableIsOfType(&hstoreUpgrade, hstoreUpgrade_t);
 	hstoreUpgrade_p = (hstoreUpgrade_t)
 		load_external_function("$libdir/hstore", "hstoreUpgrade",
 							   true, NULL);
+	AssertVariableIsOfType(&hstoreUniquePairs, hstoreUniquePairs_t);
 	hstoreUniquePairs_p = (hstoreUniquePairs_t)
 		load_external_function("$libdir/hstore", "hstoreUniquePairs",
 							   true, NULL);
+	AssertVariableIsOfType(&hstorePairs, hstorePairs_t);
 	hstorePairs_p = (hstorePairs_t)
 		load_external_function("$libdir/hstore", "hstorePairs",
 							   true, NULL);
+	AssertVariableIsOfType(&hstoreCheckKeyLen, hstoreCheckKeyLen_t);
 	hstoreCheckKeyLen_p = (hstoreCheckKeyLen_t)
 		load_external_function("$libdir/hstore", "hstoreCheckKeyLen",
 							   true, NULL);
+	AssertVariableIsOfType(&hstoreCheckValLen, hstoreCheckValLen_t);
 	hstoreCheckValLen_p = (hstoreCheckValLen_t)
 		load_external_function("$libdir/hstore", "hstoreCheckValLen",
 							   true, NULL);
@@ -65,6 +72,7 @@ _PG_init(void)
 
 /* These defines must be after the module init function */
 #define PLyObject_AsString PLyObject_AsString_p
+#define PLyUnicode_FromStringAndSize PLyUnicode_FromStringAndSize_p
 #define hstoreUpgrade hstoreUpgrade_p
 #define hstoreUniquePairs hstoreUniquePairs_p
 #define hstorePairs hstorePairs_p