[gdb/breakpoints] Fix sigsegv in info prog at exec catchpoint

[gdb/breakpoints] Fix sigsegv in info prog at exec catchpoint

With the test-case contained in this patch and compiled for debug we run into
a segfault with trunk gdb:
...
$ gdb catch-follow-exec -batch -ex "catch exec" \
  -ex "set follow-exec-mode new" -ex "run" -ex "info prog"
Catchpoint 1 (exec)
process xxx is executing new program: /usr/bin/ls
[New inferior 2 (process 0)]
[New process xxx]

Thread 2.1 "ls" hit Catchpoint 1 (exec'd /usr/bin/ls), in _start () from
  /lib64/ld-linux-x86-64.so.2
Segmentation fault (core dumped)
...

The patch fixes the segfault by returning an error in info_program_command
if get_last_target_status returns minus_one_ptid.

The test-case is non-standard, because the standard approach runs into
PR23368, a problem with gdb going to the background.

Build and reg-tested on x86_64-linux.

2018-07-26  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/23366
	* infcmd.c (info_program_command): Handle ptid == minus_one_ptid.

	* gdb.base/catch-follow-exec.c: New test.
	* gdb.base/catch-follow-exec.exp: New file.