Use CLEARPGRES() macro to call PQclear() and set res to NULL.

This simplifies some of the error handling blocks, as now
we can unconditionally use this macro without worrying about multiple
PQclear() calls causing a double-free().

Per discussion with Daniele.

bin/pgut/pgut-fe.h

@@ -73,4 +73,9 @@ extern void pgut_readopt(const char *path, pgut_option options[], int elevel);
 extern void pgut_setopt(pgut_option *opt, const char *optarg, pgut_optsrc src);
 extern bool pgut_keyeq(const char *lhs, const char *rhs);
 
+/* So we don't need to fret over multiple calls to PQclear(), e.g.
+ * in cleanup labels.
+ */
+#define CLEARPGRES(pgres)  do { PQclear(pgres); pgres = NULL; } while (0)
+
 #endif   /* PGUT_FE_H */