[WindowsDriver] Improve VSInstallPath check for IDE subdirectory

[dim]

Summary

This avoids potential memory allocation failures, if VSInstallPath is
not empty, but also does not contain the string "\Common7\IDE".

Fixes: https://github.com/llvm/llvm-project/issues/59434

[mstorsjo]

LGTM, thanks!

[mstorsjo]

(I dunno what the status is wrt tests for this codepath, whether we have testcases that mock a real setup, where these situations can be tested. If we do have that, it would of course be great to have coverage of this case too.)

llvm/lib/WindowsDriver/MSVCPaths.cpp

 
 bool findVCToolChainViaRegistry(std::string &Path, ToolsetLayout &VSLayout) {
   std::string VSInstallPath;
   if (getSystemRegistryString(R"(SOFTWARE\Microsoft\VisualStudio\$VERSION)",
                               "InstallDir", VSInstallPath, nullptr) ||
       getSystemRegistryString(R"(SOFTWARE\Microsoft\VCExpress\$VERSION)",
                               "InstallDir", VSInstallPath, nullptr)) {
     if (!VSInstallPath.empty()) {
-      SmallString<256> VCPath(StringRef(VSInstallPath.c_str(),
-                                        VSInstallPath.find(R"(\Common7\IDE)")));
+      auto pos = VSInstallPath.find(R"(\Common7\IDE)");
+      if(pos == std::string::npos)
+        return false;
+      SmallString<256> VCPath(StringRef(VSInstallPath.c_str(), pos));
       sys::path::append(VCPath, "VC");
 
       Path = std::string(VCPath.str());
       VSLayout = ToolsetLayout::OlderVS;
       return true;
     }
   }
   return false;
 }
 
 } // namespace llvm