In array_position()/array_positions(), beware of empty input array.

These functions incautiously fetched the array's first lower bound
even when the array is zero-dimensional, thus fetching the word
after the allocated array space.  While almost always harmless,
with very bad luck this could result in SIGSEGV.  Fix by adding
an early exit for empty input.

Per bug #17920 from Alexander Lakhin.

Discussion: https://postgr.es/m/17920-f7c228c627b6d02e%40postgresql.org

src/backend/utils/adt/array_userfuncs.c

@@ -1275,7 +1275,6 @@ array_position_common(FunctionCallInfo fcinfo)
 		PG_RETURN_NULL();
 
 	array = PG_GETARG_ARRAYTYPE_P(0);
-	element_type = ARR_ELEMTYPE(array);
 
 	/*
 	 * We refuse to search for elements in multi-dimensional arrays, since we
@@ -1286,6 +1285,10 @@ array_position_common(FunctionCallInfo fcinfo)
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("searching for elements in multidimensional arrays is not supported")));
 
+	/* Searching in an empty array is well-defined, though: it always fails */
+	if (ARR_NDIM(array) < 1)
+		PG_RETURN_NULL();
+
 	if (PG_ARGISNULL(1))
 	{
 		/* fast return when the array doesn't have nulls */
@@ -1300,6 +1303,7 @@ array_position_common(FunctionCallInfo fcinfo)
 		null_search = false;
 	}
 
+	element_type = ARR_ELEMTYPE(array);
 	position = (ARR_LBOUND(array))[0] - 1;
 
 	/* figure out where to start */
@@ -1425,9 +1429,6 @@ array_positions(PG_FUNCTION_ARGS)
 		PG_RETURN_NULL();
 
 	array = PG_GETARG_ARRAYTYPE_P(0);
-	element_type = ARR_ELEMTYPE(array);
-
-	position = (ARR_LBOUND(array))[0] - 1;
 
 	/*
 	 * We refuse to search for elements in multi-dimensional arrays, since we
@@ -1440,6 +1441,10 @@ array_positions(PG_FUNCTION_ARGS)
 
 	astate = initArrayResult(INT4OID, CurrentMemoryContext, false);
 
+	/* Searching in an empty array is well-defined, though: it always fails */
+	if (ARR_NDIM(array) < 1)
+		PG_RETURN_DATUM(makeArrayResult(astate, CurrentMemoryContext));
+
 	if (PG_ARGISNULL(1))
 	{
 		/* fast return when the array doesn't have nulls */
@@ -1454,6 +1459,9 @@ array_positions(PG_FUNCTION_ARGS)
 		null_search = false;
 	}
 
+	element_type = ARR_ELEMTYPE(array);
+	position = (ARR_LBOUND(array))[0] - 1;
+
 	/*
 	 * We arrange to look up type info for array_create_iterator only once per
 	 * series of calls, assuming the element type doesn't change underneath