Dodge compiler bug in Visual Studio 2013.

VS2013 apparently has a problem with taking the address of a formal
parameter in some cases.  We do that elsewhere without trouble, but
in this case the address is being passed to a subroutine that will
probably get inlined, so maybe the combination of those things is
what tickles the bug.  Anyway, introducing an extra copy of the
parameter value is enough to work around it.  Per trouble report
from Umair Shahid.

Report: <CAM184AcjqKYZSdQqBHDrnENXHhW=mXbUC46QYPJ=nAh0gUHCGA@mail.gmail.com>