Back-patch 9.4-era compiler warning fixes into older branches.

This applies portions of commits b64b5cc and b1aebbb to the older
branches, in hopes of getting -Werror builds to succeed there.  The
applied changes simply remove useless tests, eg checking an unsigned
variable to see if it is >= 0.  Recent versions of clang warn about
such tests by default.

Author: tglsfdc

contrib/pg_standby/pg_standby.c

@@ -358,7 +358,7 @@ SetWALFileNameForCleanup(void)
 	if (keepfiles > 0)
 	{
 		sscanf(nextWALFileName, "%08X%08X%08X", &tli, &log, &seg);
-		if (tli > 0 && log >= 0 && seg > 0)
+		if (tli > 0 && seg > 0)
 		{
 			log_diff = keepfiles / MaxSegmentsPerLogFile;
 			seg_diff = keepfiles % MaxSegmentsPerLogFile;

src/interfaces/libpq/fe-exec.c

@@ -2897,7 +2897,7 @@ PQoidStatus(const PGresult *res)
 
 	size_t		len;
 
-	if (!res || !res->cmdStatus || strncmp(res->cmdStatus, "INSERT ", 7) != 0)
+	if (!res || strncmp(res->cmdStatus, "INSERT ", 7) != 0)
 		return "";
 
 	len = strspn(res->cmdStatus + 7, "0123456789");
@@ -2921,7 +2921,6 @@ PQoidValue(const PGresult *res)
 	unsigned long result;
 
 	if (!res ||
-		!res->cmdStatus ||
 		strncmp(res->cmdStatus, "INSERT ", 7) != 0 ||
 		res->cmdStatus[7] < '0' ||
 		res->cmdStatus[7] > '9')