Fix NPE in ios sdk

Summary: Haven't been able to repro this, but from the stack trace it seems like this NSData object is null in some cases: https://www.internalfb.com/logview/details/instagram_ios_crashes/d0e929eae516ec48380518380cfca9f5/?selected-logview-tab=Employee+Traces

Reviewed By: lblasa

Differential Revision: D37995141

fbshipit-source-id: 937217a7a5b1c7cc992f3f9ca5dc3a7e31165d1c

iOS/FlipperKit/FlipperPlatformWebSocket.mm

@@ -60,6 +60,9 @@ static constexpr int connectionKeepaliveSeconds = 10;
   NSString* certificatePathObjC =
       [NSString stringWithUTF8String:certificatePath.c_str()];
   NSData* certificateData = [NSData dataWithContentsOfFile:certificatePathObjC];
+  if (!certificateData) {
+    return;
+  }
 
   NSString* password = [NSString stringWithUTF8String:PASSWORD];
   NSDictionary* optionsDictionary = [NSDictionary