BamHeader::tid incorrect at first call

[Peter]

It seems like BamHeader::tid is not returning a correct value first time it called, but after that it seems corrected. I have something like this

BedData(const BamHeader& hd, std::string& str)
{
std::cerr << ": " << hd.tid(str) << "\n";
std::cerr << ": " << hd.tid(str) << "\n";
...
}

which gives output

: 0
: 1

That is first of all inconsistent, and the first line is incorrect.

[Peter]

(In [3140]) fixes #776 a bug in function BamHeader::tid