jit: fix Uninitialized string offset warning being emitted

The offset is an invalid type and should return early

Author: Girgias

Zend/tests/offsets/strings/const_dimension/object_offset_behaviour.phpt

@@ -1,7 +1,7 @@
 --TEST--
 Object offset behaviour
 --XFAIL--
-Uninitialized string offset warning on read is emitted with const offset
+Object of class stdClass could not be converted to int warning with const offset in JIT
 --FILE--
 <?php
 

ext/opcache/jit/zend_jit_helpers.c

@@ -1098,6 +1098,9 @@ static zend_string* ZEND_FASTCALL zend_jit_fetch_dim_str_r_helper(zend_string *s
 	} else {
 		offset = Z_LVAL_P(dim);
 	}
+	if (UNEXPECTED(EG(exception) != NULL)) {
+		return ZSTR_EMPTY_ALLOC();
+	}
 	return zend_jit_fetch_dim_str_offset(str, offset);
 }