[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.

lib/legacy/zstd_v06.c

@@ -3501,8 +3501,10 @@ static size_t ZSTDv06_decompressSequences(
     {   size_t const lastLLSize = litEnd - litPtr;
         if (litPtr > litEnd) return ERROR(corruption_detected);   /* too many literals already used */
         if (op+lastLLSize > oend) return ERROR(dstSize_tooSmall);
-        memcpy(op, litPtr, lastLLSize);
-        op += lastLLSize;
+        if (lastLLSize > 0) {
+            memcpy(op, litPtr, lastLLSize);
+            op += lastLLSize;
+        }
     }
 
     return op-ostart;