muxread,MuxGet: add an assert

assert idx != IDX_LAST_CHUNK to quiet a static analysis warning
(reported by Coverity):
  Indexing array kChunks of size 11 with idx.

Change-Id: I5e54b5c09d23c26abbd1a7919b6f828b35fe4f75

src/mux/muxread.c

@@ -37,6 +37,7 @@
 static WebPMuxError MuxGet(const WebPMux* const mux, CHUNK_INDEX idx,
                            uint32_t nth, WebPData* const data) {
   assert(mux != NULL);
+  assert(idx != IDX_LAST_CHUNK);
   assert(!IsWPI(kChunks[idx].id));
   WebPDataInit(data);