Undefined behavior in ZSTD_decompressStream()

[embg]

When ZSTD_decompressStream() is called with outBuffer == {NULL, 0, 0}, there is undefined behavior here when 0 is added to the NULL outBuffer pointer. UBSAN reports this as "runtime error: applying zero offset to null pointer". It was discovered by folly's CompressionTest.cpp.

Creating this issue to track the following work:

• Add a test to zstd's CI which exposes the UB.
• Refactor ZSTD_decompressStream() to avoid the UB.

Note: there is no need to raise an error since the input itself is legal.