Silence another gcc 11 warning.

Silence another gcc 11 warning.

Per buildfarm and local experimentation, bleeding-edge gcc isn't
convinced that the MemSet in reorder_function_arguments() is safe.
Shut it up by adding an explicit check that pronargs isn't negative,
and by changing MemSet to memset.  (It appears that either change is
enough to quiet the warning at -O2, but let's do both to be sure.)