Catch overflow in n_cigar and give an explicit error.

Due to the use of an unsigned 16bit integer, BAM
files can only hold reads with up to 65535 CIGAR
operators. SAM does not have this limit, but the
samtools implementation (reasonably) also has the
same 16bit limit.

Prior to this fix a very long CIGAR read would
give a confusing 'unmatched CIGAR operation'
warning and then abort with a core dump.