dev-470: don't stop on bwa_fix_xref2() failures

Peter Field has sent me an example caused by an alignment bridging three
adjacent chromosomes/contigs. Bwa-mem always aligns the query to the contig
covering the middle point of the alignment. In this example, it chooses the
middle contig, which should not be aligned. This leads to weird things failing
bwa_fix_xref2(), which cannot be fixed unless we build the contig boundaries
into the FM-index.

In the old code, bwa-mem halts when bwa_fix_xref2() fails. With this commit,
bwa-mem will give a warning instead of halting.
dev-471: fixed a compiling error from last commit