[lib] Fix NULL pointer dereference

When the output buffer is `NULL` with size 0, but the frame content size
is non-zero, we will write to the NULL pointer because our bounds check
underflowed.

This was exposed by a recent PR that allowed an empty frame into the
single-pass shortcut in streaming mode.

* Fix the bug.
* Fix another NULL dereference in zstd-v1.
* Overflow checks in 32-bit mode.
* Add a dedicated test.
* Expose the bug in the dedicated simple_decompress fuzzer.
* Switch all mallocs in fuzzers to return NULL for size=0.
* Fix a new timeout in a fuzzer.

Neither clang nor gcc show a decompression speed regression on x86-64.
On x86-32 clang is slightly positive and gcc loses 2.5% of speed.

Credit to OSS-Fuzz.

tests/fuzz/stream_round_trip.c

@@ -146,10 +146,9 @@ int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
     if (neededBufSize > bufSize) {
         free(cBuf);
         free(rBuf);
-        cBuf = (uint8_t*)malloc(neededBufSize);
-        rBuf = (uint8_t*)malloc(neededBufSize);
+        cBuf = (uint8_t*)FUZZ_malloc(neededBufSize);
+        rBuf = (uint8_t*)FUZZ_malloc(neededBufSize);
         bufSize = neededBufSize;
-        FUZZ_ASSERT(cBuf && rBuf);
     }
     if (!cctx) {
         cctx = ZSTD_createCCtx();
@@ -166,7 +165,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
             ZSTD_decompressDCtx(dctx, rBuf, neededBufSize, cBuf, cSize);
         FUZZ_ZASSERT(rSize);
         FUZZ_ASSERT_MSG(rSize == size, "Incorrect regenerated size");
-        FUZZ_ASSERT_MSG(!memcmp(src, rBuf, size), "Corruption!");
+        FUZZ_ASSERT_MSG(!FUZZ_memcmp(src, rBuf, size), "Corruption!");
     }
 
     FUZZ_dataProducer_free(producer);