gdbserver crash running gdb.threads/non-ldr-exc-1.exp

This fixes a gdbserver crash when running
gdb.threads/non-ldr-exc-1.exp with "maint set target-non-stop on".
The problem is that qSymbol is called when gdbserver has
current_thread == NULL.

gdb/gdbserver/ChangeLog:
2015-11-30  Pedro Alves  <palves@redhat.com>

	* gdbthread.h (find_any_thread_of_pid): Declare.
	* inferiors.c (thread_of_pid, find_any_thread_of_pid): New
	functions.
	* server.c (handle_query): If current_thread is NULL, look for
	another thread of the selected process.