Don't output null pathname in core_target::build_file_mappings warning

Don't output null pathname in core_target::build_file_mappings warning

While looking into the regressions reported by Luis Machado, I noticed
that null pathnames were being output in the warnings.  E.g.

warning: Can't open file (null) during file-backed mapping note processing

I've changed the warning to output the pathname found in the note,
like this:

warning: Can't open file /var/lib/docker/aufs/diff/d07c...e21/lib/x86_64-linux-gnu/libc-2.27.so during file-backed mapping note processing

(I've shortened one of the path elements above.)

gdb/ChangeLog:

	* corelow.c (core_target::build_file_mappings): Don't output
	null pathname in warning.