Fix MSVC warning introduced in ea1db8ae70.

Discussion: https://postgr.es/m/CA+hUKGJR1BhCORa5WdvwxztD3arhENcwaN1zEQ1Upg20BwjKWA@mail.gmail.com
Reported-by: Thomas Munro

src/backend/utils/adt/pg_locale.c

@@ -2908,6 +2908,7 @@ icu_language_tag(const char *loc_str, int elevel)
 	ereport(ERROR,
 			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 			 errmsg("ICU is not supported in this build")));
+	return NULL;		/* keep compiler quiet */
 #endif							/* not USE_ICU */
 }
 

src/bin/initdb/initdb.c

@@ -2298,6 +2298,7 @@ icu_language_tag(const char *loc_str)
 	return langtag;
 #else
 	pg_fatal("ICU is not supported in this build");
+	return NULL;		/* keep compiler quiet */
 #endif
 }