Fix compatibility with PostgreSQL 9.x: rows affected.

Starting with PostgreSQL 9.0, libpq's PQcmdTuples() returns row count
for SELECT queries (previously it returned empty string).

Because we're using this value to detect number of changed rows,
both: "postgres_rewrite" directive and "$postgres_affected" variable
were working incorrectly.

Reported by Yichun Zhang (agentzh).

src/ngx_postgres_processor.c

@@ -353,10 +353,12 @@ ngx_postgres_process_response(ngx_http_request_t *r, PGresult *res)
     pgctx->var_rows = PQntuples(res);
 
     /* set $postgres_affected */
-    affected = PQcmdTuples(res);
-    affected_len = ngx_strlen(affected);
-    if (affected_len) {
-        pgctx->var_affected = ngx_atoi((u_char *) affected, affected_len);
+    if (ngx_strncmp(PQcmdStatus(res), "SELECT", sizeof("SELECT") - 1)) {
+        affected = PQcmdTuples(res);
+        affected_len = ngx_strlen(affected);
+        if (affected_len) {
+            pgctx->var_affected = ngx_atoi((u_char *) affected, affected_len);
+        }
     }
 
     if (pglcf->rewrites) {