[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_v02.c

@@ -2836,7 +2836,9 @@ static size_t ZSTD_getcBlockSize(const void* src, size_t srcSize, blockPropertie
 static size_t ZSTD_copyUncompressedBlock(void* dst, size_t maxDstSize, const void* src, size_t srcSize)
 {
     if (srcSize > maxDstSize) return ERROR(dstSize_tooSmall);
-    memcpy(dst, src, srcSize);
+    if (srcSize > 0) {
+        memcpy(dst, src, srcSize);
+    }
     return srcSize;
 }
 
@@ -3229,8 +3231,10 @@ static size_t ZSTD_decompressSequences(
             size_t lastLLSize = litEnd - litPtr;
             if (litPtr > litEnd) return ERROR(corruption_detected);
             if (op+lastLLSize > oend) return ERROR(dstSize_tooSmall);
-            if (op != litPtr) memmove(op, litPtr, lastLLSize);
-            op += lastLLSize;
+            if (lastLLSize > 0) {
+                if (op != litPtr) memmove(op, litPtr, lastLLSize);
+                op += lastLLSize;
+            }
         }
     }