[oops] Clear, not se, selected_frame_p

[Andrew Cagney]

"info frame" gives subtlely different output dependant on the command 
variant:

(gdb) info frame
Stack level 0, frame at 0x7ffff770:
  ...
(gdb) info frame 0
Stack frame at 0x7ffff770:
  ...

this tipo fix restores the behavior (savedregs.exp detects this).

committed,
Andrew