[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/fuzz_helpers.h

@@ -56,6 +56,17 @@ extern "C" {
 #define FUZZ_STATIC static
 #endif
 
+/**
+ * malloc except return NULL for zero sized data and FUZZ_ASSERT
+ * that malloc doesn't fail.
+ */
+void* FUZZ_malloc(size_t size);
+
+/**
+ * memcmp but accepts NULL.
+ */
+int FUZZ_memcmp(void const* lhs, void const* rhs, size_t size);
+
 #ifdef __cplusplus
 }
 #endif