Detect setlocale(LC_CTYPE, NULL) clobbering previous return values.

POSIX permits setlocale() calls to invalidate any previous setlocale()
return values.  Commit 5f538ad004
neglected to account for that.  In advance of fixing that bug, switch to
failing hard on affected configurations.  This is a planned temporary
commit to assay buildfarm-represented configurations.

src/backend/utils/adt/pg_locale.c

@@ -58,6 +58,7 @@
 #include "catalog/pg_collation.h"
 #include "catalog/pg_control.h"
 #include "mb/pg_wchar.h"
+#include "utils/builtins.h"
 #include "utils/hsearch.h"
 #include "utils/memutils.h"
 #include "utils/pg_locale.h"
@@ -148,6 +149,7 @@ pg_perm_setlocale(int category, const char *locale)
 	char	   *result;
 	const char *envvar;
 	char	   *envbuf;
+	char		orig_result[LC_ENV_BUFSIZE];
 
 #ifndef WIN32
 	result = setlocale(category, locale);
@@ -173,6 +175,7 @@ pg_perm_setlocale(int category, const char *locale)
 
 	if (result == NULL)
 		return result;			/* fall out immediately on failure */
+	strlcpy(orig_result, result, sizeof(orig_result));
 
 	/*
 	 * Use the right encoding in translated messages.  Under ENABLE_NLS, let
@@ -231,6 +234,17 @@ pg_perm_setlocale(int category, const char *locale)
 	}
 
 	snprintf(envbuf, LC_ENV_BUFSIZE - 1, "%s=%s", envvar, result);
+	if (strcmp(orig_result, result) != 0)
+	{
+		char		hex[2 * LC_ENV_BUFSIZE + 1];
+		unsigned	hexlen;
+
+		hexlen =
+			hex_encode(result, Min(1 + strlen(result), LC_ENV_BUFSIZE), hex);
+		hex[hexlen] = '\0';
+		elog(FATAL, "setlocale() result %s clobbered to 0x%s",
+			 orig_result, hex);
+	}
 
 	if (putenv(envbuf))
 		return NULL;