Define META_FREE in a way that doesn't cause -Wempty-body warnings.

That get rids of the only -Wempty-body warning when compiling postgres
with gcc 4.8/9. As 6550b901 shows, it's useful to be able to use that
option routinely.

Without asserts there's many more warnings, but that's food for
another commit.

contrib/fuzzystrmatch/dmetaphone.c

@@ -195,7 +195,7 @@ dmetaphone_alt(PG_FUNCTION_ARGS)
  * in a case like this.
  */
 
-#define META_FREE(x)			/* pfree((x)) */
+#define META_FREE(x) ((void)true) /* pfree((x)) */
 #else							/* not defined DMETAPHONE_MAIN */
 
 /* use the standard malloc library when not running in PostgreSQL */