Avoid crash when calling warning too early

I noticed that if you pass the name of an existing file (not a
directory) as the argument to --data-directory, gdb will crash:

    $ ./gdb -nx  --data-directory  ./gdb
    ../../binutils-gdb/gdb/target.c:590:56: runtime error: member call on null pointer of type 'struct target_ops'

This was later reported as PR gdb/23838.

This happens because warning ends up calling
target_supports_terminal_ours, which calls current_top_target, which
returns nullptr this early.

This fixes the problem by handling this case specially in
target_supports_terminal_ours.  I also changed
target_supports_terminal_ours to return bool.

gdb/ChangeLog
2018-11-08  Tom Tromey  <tom@tromey.com>

	PR gdb/23555:
	PR gdb/23838:
	* target.h (target_supports_terminal_ours): Return bool.
	* target.c (target_supports_terminal_ours): Handle case where
	current_top_target returns nullptr.  Return bool.

gdb/testsuite/ChangeLog
2018-11-08  Tom Tromey  <tom@tromey.com>

	PR gdb/23555:
	PR gdb/23838:
	* gdb.base/warning.exp: New file.