Fix up the error processing in VG_(expand_file_name). E.g. giving

Fix up the error processing in VG_(expand_file_name). E.g. giving
--log-file=  on the command line results in the following error:

valgrind: --log-file: filename is emptyBad option: --log-file=
...

Relatedly, fix the 1st argument to VG_(expand_file_name) in coredump-elf.c.
This should not contain additional verbiage as it is assumed to be an option
name which us used to construct an error message containing
option_name=file_name

As an aside, this logic in coredump-elf.c seems odd:
If VG_(clo_log_fname_expanded) is not NULL, then it has already been
expanded in main_process_cmd_line_options. Expanding it again would only
make a difference, if the original logfile name contained an environment
variable whose value contained %q{whatever} thereby referring to a yet
another environment variable. That seems strange.
But I'm not touching it.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14794