Allow - in %p for printf

Allow - in %p for printf

PR cli/19918 points out that a printf format like "%-5p" will cause a
gdb crash.  The bug is problem is that printf_pointer doesn't take the
"-" flag into account.

gdb/ChangeLog
2018-03-14  Tom Tromey  <tom@tromey.com>

	PR cli/19918:
	* printcmd.c (printf_pointer): Allow "-" in format.

gdb/testsuite/ChangeLog
2018-03-14  Tom Tromey  <tom@tromey.com>

	PR cli/19918:
	* gdb.base/printcmds.exp (test_printf): Add printf test using '-'
	flag.