[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/zstd_frame_info.c

@@ -22,6 +22,9 @@
 int LLVMFuzzerTestOneInput(const uint8_t *src, size_t size)
 {
     ZSTD_frameHeader zfh;
+    if (size == 0) {
+        src = NULL;
+   }
     /* You can fuzz any helper functions here that are fast, and take zstd
      * compressed data as input. E.g. don't expect the input to be a dictionary,
      * so don't fuzz ZSTD_getDictID_fromDict().