This problem was found by a colleague while using GDB with the ADS tools.

These tools do not set the DW_AT_comp_dir attribute in a compilation unit 
entry, so GDB will crash trying to reference a NULL pointer.

Regardless of whether ADS is correct not to generate this directive (the 
Dwarf spec appears not to require it to), GDB should not be falling over 
in this way.  The following patch fixes the problem:

<date>  Ian Rickards  <irickard@arm.com>

	* dwarf2.c (concat_filename): If we can't establish the directory 
	just return the filename.