utils.c: Raise exceptions from `convTHING' with null arguments.

This can happen as a result of using `convTHING' in an attribute `set'
function, and the Python program trying to `del' the attribute.
Unfortunately, these conversion functions are already being used in this
context, and it leads to segfaults, e.g., from

  del C.Key(C.KeyFile('', C.KOPEN_WRITE | C.KOPEN_NOFILE), 0, 'k').exptime

Easy fix.