merge_tests 102 FAIL due to libsvn_diff prefix/suffix scanning

merge_tests 102 is failing on one of the buildbots. I get a valgrind
error about uninitialised memory in libsvn_diff:

==7411== Conditional jump or move depends on uninitialised value(s)
==7411== at 0x579E56A: datasources_open (diff_file.c:787)
==7411== by 0x579AC86: svn_diff_diff3_2 (diff3.c:276)
==7411== by 0x579F694: svn_diff_file_diff3_2 (diff_file.c:1263)
==7411== by 0x50F146F: do_text_merge (merge.c:406)
==7411== by 0x50F2B33: merge_text_file (merge.c:984)
==7411== by 0x50F3DC6: svn_wc__internal_merge (merge.c:1390)
==7411== by 0x50F4294: svn_wc_merge4 (merge.c:1484)
==7411== by 0x4E69ADC: merge_file_changed (merge.c:1514)
==7411== by 0x4E74EF2: do_file_merge (merge.c:6922)
==7411== by 0x4E79030: do_merge (merge.c:8751)
==7411== by 0x4E7E8E3: merge_peg_locked (merge.c:10746)
==7411== by 0x4E7E9F8: merge_peg_cb (merge.c:10781)

It's the prefix/suffix scanning code.

-- 
Philip