Closes PECL #5944; binding an invalid parameter can lead to segfaults.

Modified patch provided by curt@php.net.

ext/pdo/pdo_stmt.c

@@ -328,7 +328,17 @@ static int really_register_bound_param(struct pdo_bound_param_data *param, pdo_s
 	
 	/* tell the driver we just created a parameter */
 	if (stmt->methods->param_hook) {
-		return stmt->methods->param_hook(stmt, pparam, PDO_PARAM_EVT_ALLOC TSRMLS_CC);
+		if (!stmt->methods->param_hook(stmt, pparam,
+				PDO_PARAM_EVT_ALLOC TSRMLS_CC)) {
+			/* driver indicates that the parameter doesn't exist.
+			 * remove it from our hash */
+			if (pparam->name) {
+				zend_hash_del(hash, pparam->name, pparam->namelen);
+			} else {
+				zend_hash_index_del(hash, pparam->paramno);
+			}
+			return 0;
+		}
 	}
 
 	return 1;