reverse-finish: turn internal error into normal error

reverse-finish: turn internal error into normal error

The reverse-finish command results in an internal error if it cannot determine
the current function.

  (gdb) c
  Continuing.

  Program received signal SIGSEGV, Segmentation fault.
  0x0000000000000000 in ?? ()
  (gdb) reverse-finish
  Run back to call of #0  0x0000000000000000 in ?? ()
  gdb/infcmd.c:1576: internal-error: Finish: couldn't find function.
  A problem internal to GDB has been detected,
  further debugging may prove unreliable.
  Quit this debugging session? (y or n) y

This is not an internal error case since the command may be used in scenarios
where there is no function at the current PC, e.g. after calling through a bad
function pointer.

Turn this into a normal error.

gdb/
	* infcmd.c (finish_backward): Turn internal error into normal error.

testsuite/
	* gdb.btrace/segv.c: New.
	* gdb.btrace/segv.exp: New.