Fix possible bug when no args have been provided to the executable

Hi,

This bug is related to:

  <https://sourceware.org/ml/gdb-patches/2017-06/msg00216.html>

On stringify_argv, we have to check if args[0] is not NULL before
stringifying anything, otherwise we might do the wrong thing when
trimming the "ret" string in the end.  args[0] will be NULL when no
arguments are passed to the inferior that will be started.

Checked in as obvious.

gdb/ChangeLog:
2017-06-08  Sergio Durigan Junior  <sergiodj@redhat.com>

	* common/common-utils.c (stringify_argv): Check for "arg[0] !=
	NULL".