[ VM ] Fix invalid free introduced in c42c76f590

It's possible for loading the CLI isolate from snapshot to be skipped
without failing for SIMARM, so *error would not point to a valid error,
causing free(*error) to crash.

TEST=Existing tests

Change-Id: Ic178f7bd0f89c4440b2daf714fbb2e428867bd72
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/178723
Reviewed-by: Ben Konyi <bkonyi@google.com>

runtime/bin/main.cc

@@ -592,7 +592,10 @@ static Dart_Isolate CreateAndSetupDartDevIsolate(const char* script_uri,
   IsolateGroupData* isolate_group_data = nullptr;
   IsolateData* isolate_data = nullptr;
 
-  AppSnapshot* app_snapshot;
+  if (error != nullptr) {
+    *error = nullptr;
+  }
+  AppSnapshot* app_snapshot = nullptr;
   bool isolate_run_app_snapshot = true;
   if (dartdev_path.get() != nullptr &&
       (app_snapshot = Snapshot::TryReadAppSnapshot(dartdev_path.get())) !=
@@ -618,8 +621,10 @@ static Dart_Isolate CreateAndSetupDartDevIsolate(const char* script_uri,
     isolate_run_app_snapshot = false;
     dartdev_path = DartDevIsolate::TryResolveDartDevKernelPath();
     // Clear error from app snapshot and retry from kernel.
-    free(*error);
-    *error = nullptr;
+    if (*error != nullptr) {
+      free(*error);
+      *error = nullptr;
+    }
 
     if (app_snapshot != nullptr) {
       delete app_snapshot;