parsers: fix infinite loop or out-of-bound read in fm1readmarkers (issue4888) The issue4888 was caused by 0-length obsolete marker. If msize is zero, fm1readmarkers() never ends. This patch adds several bound checks to fm1readmarker(). Therefore, 0-length and invalid-size marker should be rejected.

parsers: read sizes of metadata pair of obsolete marker at once This will make it easy to implement bound checking. Currently fm1readmarker() has no protection for corrupted obsstore and can cause infinite loop or out-of-bound reads.

filemerge: clean up temp files in a finally block This isn't really a big deal because the temp files are created in $TMPDIR, but it makes some upcoming work simpler.

check-code: detect and ban 'util.Abort' We have seen the light, please use the new way.