gdb: fix command lookup in execute_command ()

Commit b5661ff2 ("gdb: fix possible use-after-free when
executing commands") used lookup_cmd_exact () to lookup
command again after its execution to avoid possible
use-after-free error.

However this change broke test gdb.base/define.exp which
defines a post-hook for subcommand ("target testsuite").
In this case,  lookup_cmd_exact () returned NULL because
there's no command 'testsuite' in top-level commands.

This commit fixes this case by looking up the command again
using the original command line via lookup_cmd ().

Approved-By: Simon Marchi <simon.marchi@efficios.com>