i#1839: fix mcontext clobbered by dr_set_mcontext
i#1839: fix mcontext clobbered by dr_set_mcontext

Fixes a bug in dr_set_mcontext to avoid clobbering the input mcontext

Fixes #1839

Review-URL: https://codereview.appspot.com/276660043

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio