Fix build break on VS2019 due to implicit wchar_t->char conversion

Converting std::wstring to std::string by constructing the latter with
wchar_t iterators result in implicit conversion of wchar_t to char which
trips Visual Studio 2019 compiler into throwing a warning of potential
data loss during conversion. Use conv() helper function to do the
conversion and please the compiler at the same time.

renderdoccmd/renderdoccmd_win32.cpp

@@ -541,8 +541,7 @@ struct CrashHandlerCommand : public Command
         }
         else
         {
-          report += "  \"" + std::string(name.begin(), name.end()) + "\": \"" +
-                    std::string(val.begin(), val.end()) + "\",\n";
+          report += "  \"" + conv(name) + "\": \"" + conv(val) + "\",\n";
         }
       }