Fix null pointer dereference in DependencyInfoDumpingHandler

[YLChen-007]

Description

This PR fixes a null pointer dereference vulnerability (SIGSEGV) in DependencyInfoDumpingHandler::DumpDependencyInfo() that occurs when glslc is invoked with the -MD flag, and the resulting dependency info output file cannot be opened for writing.

Root Cause

When attempting to generate the .d dependency file in a read-only directory, a non-existent path, or when the disk is full, the utility function shaderc_util::GetOutputStream() correctly logs an error and returns nullptr.

However, in glslc/src/dependency_info.cc, the stream pointer dep_file_stream was dereferenced unconditionally without a null check:

*dep_file_stream << dep_string_stream.str();

This resulted in an immediate segmentation fault (SIGSEGV) and a process crash.

Fix

Added a standard null check to verify dep_file_stream before writing to it. If it is null, the handler gracefully returns false. This aligns perfectly with the identical mitigation pattern previously established in glslc/src/file_compiler.cc (e.g. Commit 1d9790184b2e8fb726719deac80caaf6374daed7).

Testing

We have reproduced the crash locally and verified that with this patch, glslc now correctly exits with a graceful error output (exit code 1) instead of crashing when facing unwritable output dependency paths.

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[dneto0]

The logic looks ok.

Please add a test to https://github.com/google/shaderc/blob/main/glslc/test/option_dash_M.py

Also, please sign the CLA. I can't accept the changes without the CLA.

[dneto0]

Thanks!