ext/pdo_odbc: Require non-empty string when building string buffer (php#21652)

A buggy driver could do this and get PDO_ODBC stuck in this loop.
Require a non-empty string, so an empty one breaks like the SQL_NO_DATA
case.

Fixes phpGH-21534

Author: NattyNarwhal

ext/pdo_odbc/odbc_stmt.c

@@ -710,7 +710,7 @@ static int odbc_stmt_get_col(pdo_stmt_t *stmt, int colno, zval *result, enum pdo
 					str = zend_string_realloc(str, used + 256, 0);
 					memcpy(ZSTR_VAL(str) + used, buf2, 256);
 					used = used + 255;
-				} else if (rc==SQL_SUCCESS) {
+				} else if (rc == SQL_SUCCESS && C->fetched_len != 0) {
 					str = zend_string_realloc(str, used + C->fetched_len, 0);
 					memcpy(ZSTR_VAL(str) + used, buf2, C->fetched_len);
 					used = used + C->fetched_len;