use WebPSafe[CM]alloc/WebPSafeFree instead of [cm]alloc/free

there's still some malloc/free in the external example
This is an encoder API change because of the introduction
of WebPMemoryWriterClear() for symmetry reasons.

The MemoryWriter object should probably go in examples/ instead
of being in the main lib, though.
mux_types.h stil contain some inlined free()/malloc() that are
harder to remove (we need to put them in the libwebputils lib
and make sure link is ok). Left as a TODO for now.

Also: WebPDecodeRGB*() function are still returning a pointer
that needs to be free()'d. We should call WebPSafeFree() on
these, but it means exposing the whole mechanism. TODO(later).

Change-Id: Iad2c9060f7fa6040e3ba489c8b07f4caadfab77b

src/enc/webpenc.c

@@ -269,7 +269,7 @@ static int DeleteVP8Encoder(VP8Encoder* enc) {
     VP8EncDeleteLayer(enc);
 #endif
     VP8TBufferClear(&enc->tokens_);
-    free(enc);
+    WebPSafeFree(enc);
   }
   return ok;
 }