Magic fuzz

[embg]

Fuzzer coverage checks

I made sure that each of these changes in zstd_decompress.c individually cause the fuzzer to crash:

-        if (dctx->format == ZSTD_f_zstd1 && ZSTD_isLegacy(src, srcSize)) {
+        if (/*dctx->format == ZSTD_f_zstd1 &&*/ ZSTD_isLegacy(src, srcSize)) {

-        if (dctx->format == ZSTD_f_zstd1 && srcSize >= 4) {
+        if (/*dctx->format == ZSTD_f_zstd1 &&*/ srcSize >= 4) {

-                size_t const cSize = ZSTD_findFrameCompressedSize_advanced(istart, (size_t)(iend-istart), zds->format);
+                size_t const cSize = ZSTD_findFrameCompressedSize(istart, (size_t)(iend-istart));

-            if (zds->format == ZSTD_f_zstd1
-                && (MEM_readLE32(zds->headerBuffer) & ZSTD_MAGIC_SKIPPABLE_MASK) == ZSTD_MAGIC_SKIPPABLE_START) {  /* skippable frame */
+            if ((MEM_readLE32(zds->headerBuffer) & ZSTD_MAGIC_SKIPPABLE_MASK) == ZSTD_MAGIC_SKIPPABLE_START) {  /* skippable frame */