Cygwin: strptime: make sure to fail on invalid input digits

Cygwin: strptime: make sure to fail on invalid input digits

conv_num returns NULL if the input is invalid, e. g., the
numbers are out of range.  However, the code fails to test
this in a lot of places.

Rather than adding checks all over the place, rename conv_num
to __conv_num and create a wrapper macro conv_num to perform
the task of error checking.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>