[Images] Bail out when GIF data is in unexpected format instead of cr…

…ashing

Summary:
When the GIF data is not a string nor NSData, `imageSource` is some unknown value set to whatever memory was on the stack. Calling `CGImageSourceGetType` on it has undefined behavior, and if it makes through to the CFRelease (maybe because it is NULL) then CFRelease will crash. This fixes a clang warning.
Closes facebook#77
Github Author: James Ide <ide@jameside.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

ReactKit/Base/RCTConvert.m

@@ -446,6 +446,9 @@ + (CAKeyframeAnimation *)GIF:(id)json
     }
 
     imageSource = CGImageSourceCreateWithData((CFDataRef)data, NULL);
+  } else {
+      RCTLogMustFix(@"Expected NSString or NSData for GIF, received %@: %@", [json class], json);
+      return nil;
   }
 
   if (!UTTypeConformsTo(CGImageSourceGetType(imageSource), kUTTypeGIF)) {