whom.c: Don't call rename(2) with source that's uninitialised.

The intent is that two rename(2)s take place: to move a file to a
backup and then return it.  The first test controlled the generation of
the destination in the char array `backup' and calling the first
rename().  The second test used that same array for the restoring
rename(), but used a different test and so could run when the first
rename hadn't, and with an uninitialised array for the source filename.

The initialisation of `backup' and the first rename needed both
environment variables `mhdist' and `mhaltmsg' to be non-empty, and the
former to have a non-zero atoi(3) value.  The second rename ignored
`mhaltmsg'.  Bug pre-dates git.