Harden public C and C++ APIs and Source/Sink against null pointers and underflow

[filtede98]

Summary

This patch hardens Snappy public C API entry points, C++ compression and decompression routines, and Source and Sink implementations against null pointer dereferences and unsigned integer underflow.

Detailed Changes

1. Public C API (snappy-c.cc):

   • Added validation in snappy_compress, snappy_compress_with_level, snappy_uncompress, snappy_uncompressed_length, and snappy_validate_compressed_buffer ensuring that input, output buffer, and length pointers are valid when lengths are positive.
   • Return SNAPPY_INVALID_INPUT when invalid null pointers are supplied.

2. Source and Sink Safety (snappy-sinksource.h, snappy-sinksource.cc):

   • Prevent unsigned underflow in ByteArraySource::Skip(size_t n) by clamping
     to left_.
   • Initialize left_ to 0 in ByteArraySource constructor if the underlying data pointer is null.
   • Guard len pointer in ByteArraySource::Peek(size_t* len) before assignment.
   • Guard deleter function pointer before invocation and �llocated_size pointer before assignment in Sink::AppendAndTakeOwnership, Sink::GetAppendBufferVariable, UncheckedByteArraySink::AppendAndTakeOwnership, and UncheckedByteArraySink::GetAppendBufferVariable.

3. C++ API Pointer Guards (snappy.cc):

   • Added null checks in GetUncompressedLength (both string and Source* overloads) returning false on null pointers.
   • Added null checks in Compress and InternalCompress returning 0 when reader, writer, or context pointer is null.
   • Added null checks in RawCompress overloads ensuring *compressed_length is cleared to 0 and execution safely returns when invalid pointers are passed.
   • Added null checks in RawUncompress, Uncompress, and UncompressAsMuchAsPossible ensuring safe returns without dereferencing null pointers.

4. Unit Test Coverage (snappy_unittest.cc):

   • Added NullPointerAndBoundsGuards test verifying safe returns and error codes across C and C++ entry points when passed null pointers or excessive skip counts.