Hold on to error message in LinkTaskMtl as C++ string. Make a copy of the incoming C string because the error message might be dynamically allocated and deallocated by the caller. Bug: angleproject:370425451 Change-Id: If4aaa93a90a1da8bc60f7839e29b705b0d2864e5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5938484 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org> Auto-Submit: Kenneth Russell <kbr@chromium.org>