Fix crashes when unbind_for_thread_switch signals an error.

Fix crashes when unbind_for_thread_switch signals an error.

 src/eval.c (unbind_for_thread_switch): Accept a 'struct
 thread_state *' argument and use specpdl_ptr and specpdl of that
 thread.  Fixes crashes if find_symbol_value signals an error.
 src/thread.c (post_acquire_global_lock): Update current_thread
 before calling unbind_for_thread_switch.  Pass the previous thread
 to unbind_for_thread_switch.