WebPAnimEncoderNewInternal: remove some unnecessary inits

enc is allocated with WebPSafeCalloc so there's no need to clear the
pointers afterward.
this has the side-effect of removing a non-inclusive term.

Bug: webp:507
Change-Id: I82f82954936638c4c15d33b2d6f0497a6a13571f

src/mux/anim_encode.c

@@ -248,9 +248,6 @@ WebPAnimEncoder* WebPAnimEncoderNewInternal(
 
   enc = (WebPAnimEncoder*)WebPSafeCalloc(1, sizeof(*enc));
   if (enc == NULL) return NULL;
-  // sanity inits, so we can call WebPAnimEncoderDelete():
-  enc->encoded_frames_ = NULL;
-  enc->mux_ = NULL;
   MarkNoError(enc);
 
   // Dimensions and options.