prevent step_thread test from infinite looping

If an internal rr error occurs during the step_thread test, the
gdb_rr.log file will start to fill up with:

(gdb) s
s
[ERROR /home/froydnj/src/rr/src/Registers.cc:289:maybe_print_reg_mismatch() errno: 0 'Success']
 -> r11 0x306 != 0x206 (replaying vs. recorded)
Remote connection closed
(gdb) s
s
The program is not being run.
(gdb) s
s
The program is not being run.
(gdb) s

...and so forth.  Short-circuit this by checking to see whether gdb died
when we issued a step command.