[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/simple_decompress.c

@@ -35,8 +35,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
     }
 
     size_t const bufSize = FUZZ_dataProducer_uint32Range(producer, 0, 10 * size);
-    void *rBuf = malloc(bufSize);
-    FUZZ_ASSERT(rBuf);
+    void *rBuf = FUZZ_malloc(bufSize);
 
     ZSTD_decompressDCtx(dctx, rBuf, bufSize, src, size);
     free(rBuf);