Fix PR cli/23785: Check if file exists when invoking "restore FILE binary"

Fix PR cli/23785: Check if file exists when invoking "restore FILE binary"

This simple patch fixes the segfault reported on PR cli/23785, which
happens when using the "restore FILE binary" command with a
non-existent file.  We just have to check if the file handler returned
by "gdb_fopen_cloexec" is not NULL, and error out if it is.

A test has also been added to gdb.base/restore.exp in order to
exercise this scenario.

No regressions introduced.

gdb/ChangeLog:
2018-10-18  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR cli/23785
	* cli/cli-dump.c (restore_binary_file): Check if "file" is
	NULL.

gdb/testsuite/ChangeLog:
2018-10-18  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR cli/23785
	* gdb.base/restore.exp: New test to check if "restore" with an
	invalid file doesn't segfault.