blame can access NULL-pointer

Hi,

In file subversion\libsvn_client\blame.c, line 684 there is an assert:

   /* The callback has to have been called at least once. */
   assert(frb.last_filename != NULL);

I just got a crash report for TSVN where this assert is being hit: the
user passed a revision range which didn't touch the file to blame. So
the callback was never hit.

I suggest returning an error there, something like "nothing to blame for
the specified revision range".

Stefan

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