bug#5060: More debugging information

[Michael Welsh Duggan]

Shortly before the failure occurs, gdb-frame-handler is called.  In the
context of the failure, the frame variable is set to:

  ((line . "29") (file . "longjmp.c") (func . "__libc_siglongjmp") 
   (addr . "0xb79ef6c1") (level . "0"))

gdb-selected-file is nil, since there is no 'fullname.
gdb-selected-line gets 29.  So, gud-last-frame is '(nil . 29).
gud-display-frame is then called, which calls gud-display-line with the
args from gud-last frame.  I.e., (gud-display-frame nil 29).  This nil
is what is later causing problems when gud-find-file is called.

My guess is that gud-display-frame should not be called if
gdb-selected-file is nil.  But I do not understand the code well enough
to know that this is the case.

-- 
Michael Welsh Duggan
(md5i@md5i.com)