Fix null pointer dereference in DependencyInfoDumpingHandler

Add a null check for the return value of GetOutputStream() in
dependency_info.cc before dereferencing the stream pointer. When
GetOutputStream() fails to open the output file (e.g., due to
permission errors, full disk, or non-existent directory), it returns
nullptr. The code previously unconditionally dereferenced this pointer,
causing a segmentation fault (SIGSEGV).

This is the same vulnerability pattern that was fixed in
file_compiler.cc (commit 1d97901), but was missed in the
dependency_info.cc code path. The fix follows the identical pattern:
check the pointer for null before use and return false on failure.

Bug: Null pointer dereference when glslc is invoked with -MD flag
and the dependency info output file cannot be opened for writing.

Author: YLChen-007

glslc/src/dependency_info.cc

@@ -50,6 +50,10 @@ bool DependencyInfoDumpingHandler::DumpDependencyInfo(
     std::ofstream potential_file_stream_for_dep_info_dump;
     std::ostream* dep_file_stream = shaderc_util::GetOutputStream(
         dep_file_name, &potential_file_stream_for_dep_info_dump, &std::cerr);
+    if (!dep_file_stream) {
+      // An error message has already been emitted to the stderr stream.
+      return false;
+    }
     *dep_file_stream << dep_string_stream.str();
     if (dep_file_stream->fail()) {
       std::cerr << "glslc: error: error writing dependent_files info to output "

glslc/test/option_dash_M.py

@@ -752,3 +752,13 @@ class TestErrorMissingDependencyTargetName(expect.StderrMatch):
     glslc_args = ['target', 'shader.vert', '-c', '-MT']
     expected_stderr = ['glslc: error: '
                        'missing dependency info target after \'-MT\'\n']
+
+
+@inside_glslc_testsuite('OptionsCapM')
+class TestErrorDashMFUnwritableFile(expect.ErrorMessageSubstr):
+    """Tests that when we fail to make an output dependency file, glslc
+    fails gracefully and emits an error message instead of crashing."""
+    environment = EMPTY_SHADER_IN_CURDIR
+    bad_file = '/file/should/not/exist/today.d'
+    glslc_args = ['-MD', '-MF', bad_file, 'shader.vert']
+    expected_error_substr = ['cannot open output file']