fix: add fork failure check in integration test

Co-authored-by: aider (deepseek/deepseek-reasoner) <aider@aider.chat>

tests/integration.c

@@ -39,6 +39,10 @@ int main(void) {
 
     /* 2. start the looper */
     pid_t pid = fork();
+    if (pid < 0) {
+        perror("fork");
+        return 1;
+    }
     if (pid == 0) {
         execl("./looper", "looper", NULL);
         perror("execl");