Tweak a couple of macros in the regex code to suppress compiler warnings

from "clang".  The VERR changes make an assignment unconditional, which is
probably easier to read/understand anyway, and one can hardly argue that
it's worth shaving cycles off the case of reporting another error when
one has already been detected.  The INSIST change limits where that macro
can be used, but not in a way that creates a problem for any existing call.