gdb: fix thread_step_over_chain_length

If I debug a single-thread program and look at the infrun debug logs, I
see:

    [infrun] start_step_over: stealing global queue of threads to step, length = 2

That makes no sense... turns out there's a buglet in
thread_step_over_chain_length, "num" should be initialized to 0.  I
think this bug is a leftover from an earlier version of the code (not
merged upstream) that manually walked the list, where the first item was
implicitly counted (hence the 1).

Change-Id: I0af03aa93509aed36528be5076894dc156a0b5ce