WebPMuxGetChunk: add an assert

assert idx != IDX_LAST_CHUNK to quiet a static analysis warning
(reported by Coverity):
  overrun-local: Overrunning array kChunks of 11 12-byte elements at
    element index 11 (byte offset 143) using index idx (which evaluates
    to 11).

Change-Id: I85ae159e338d5670c007f76277ffe94e7a1dcafc

src/mux/muxread.c

@@ -432,6 +432,7 @@ WebPMuxError WebPMuxGetChunk(const WebPMux* mux, const char fourcc[4],
     return WEBP_MUX_INVALID_ARGUMENT;
   }
   idx = ChunkGetIndexFromFourCC(fourcc);
+  assert(idx != IDX_LAST_CHUNK);
   if (IsWPI(kChunks[idx].id)) {     // An image chunk.
     return WEBP_MUX_INVALID_ARGUMENT;
   } else if (idx != IDX_UNKNOWN) {  // A known chunk type.