Handle parse number error in goto_bookmark_command

Handle parse number error in goto_bookmark_command

In GDB mainline, the error message for goto-bookmark
isn't perfect.

 (gdb) goto-bookmark 1.1
 goto-bookmark: no bookmark found for ''.

This patch tweaks the error message by checking the return value of
get_number.  With patch applied, it becomes:

 (gdb) goto-bookmark 1.1
 goto-bookmark: invalid bookmark number '1.1'.

gdb:

2014-03-06  Yao Qi  <yao@codesourcery.com>

	* reverse.c (goto_bookmark_command): Add local 'p'.  Emit error
	early if get_number returns zero.  Use 'p' instead of 'args'.