Backport fix from commit 125d817

This fixes a read of unitialized memory.
Full commit hash: 125d81774fe87a2bc18023d999d8e510678c38fb.

lib/legacy/zstd_v05.c

@@ -2944,6 +2944,7 @@ size_t ZSTDv05_decodeLiteralsBlock(ZSTDv05_DCtx* dctx,
         {
             size_t litSize, litCSize, singleStream=0;
             U32 lhSize = ((istart[0]) >> 4) & 3;
+            if (srcSize < 5) return ERROR(corruption_detected);   /* srcSize >= MIN_CBLOCK_SIZE == 3; here we need up to 5 for case 3 */
             switch(lhSize)
             {
             case 0: case 1: default:   /* note : default is impossible, since lhSize into [0..3] */