Exercise ZSTD_findDecompressedSize() in the simple decompression fuzzer (#3959)

* Improve decompression fuzzer

* Fix legacy frame header fuzzer crash, add unit test

tests/fuzz/simple_decompress.c

@@ -37,7 +37,13 @@ int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
     size_t const bufSize = FUZZ_dataProducer_uint32Range(producer, 0, 10 * size);
     void *rBuf = FUZZ_malloc(bufSize);
 
-    ZSTD_decompressDCtx(dctx, rBuf, bufSize, src, size);
+    size_t const dSize = ZSTD_decompressDCtx(dctx, rBuf, bufSize, src, size);
+    if (!ZSTD_isError(dSize)) {
+        /* If decompression was successful, the content size from the frame header(s) should be valid. */
+        size_t const expectedSize = ZSTD_findDecompressedSize(src, size);
+        FUZZ_ASSERT(expectedSize != ZSTD_CONTENTSIZE_ERROR);
+        FUZZ_ASSERT(expectedSize == ZSTD_CONTENTSIZE_UNKNOWN || expectedSize == dSize);
+    }
     free(rBuf);
 
     FUZZ_dataProducer_free(producer);