This commit fixes GTM bug as reported by Nikhil Sontakke:

If the GTM Proxy wants multiple GXIDs, it sends a MSG_TXN_BEGIN_GETGXID_MULTI
message to the GTM. It sends a transaction count in that message.

The calculation of the range is off by one. It's perfectly possible that the
proxy asks for just 1 gxid. PFA, a patch to fix that. Luckily it causes no
other side effects AFAICS. Can avoid this for earlier branches.

He kindly provided the fix.