r368: bugfix - wrong CIGAR when bridging 3 contigs

In this case, bwa_fix_xref() will return insane coordinates. The old version
did not check the return status and write wrong CIGAR. This bug only happen to
very short assembly contigs.
r369: bugfix - segfault caused by the last change

Sigh... Even the simplest change can lead to new bugs.
Merge branch 'master' into master_fixes