"enable count" user input error handling (PR gdb/15678)

"enable count" user input error handling (PR gdb/15678)

Typing "enable count" by itself crashes GDB. Also, if you omit the
breakpoint number/range, the error message is not very clear:

(gdb) enable count 2
warning: bad breakpoint number at or near ''
(gdb) enable count
Segmentation fault (core dumped)

With this patch, the error messages are slightly more helpful:

(gdb) enable count 2
Argument required (one or more breakpoint numbers).
(gdb) enable count
Argument required (hit count).

gdb/ChangeLog:

	PR gdb/15678
	* breakpoint.c (map_breakpoint_numbers): Check for empty args
	string.
	(enable_count_command): Check args for NULL value.

gdb/testsuite/ChangeLog:

	PR gdb/15678
	* gdb.base/ena-dis-br.exp: Test "enable count" for bad user input.