Help avoid overflow on arithmetic involving POS, MPOS and TLEN
by limiting values in the SAM parser to fit in 62 bits (or 63
for TLEN as it's signed).  The new limit is still massively bigger
than any known reference so it should not cause any problems
in practice.

Credit to OSS-Fuzz
Fixes oss-fuzz 68750

Limiting POS to 62 bits helps avoid the risk of signed overflow
when it's set to a very extreme value.  The maximum is still
much higher than the length of the longest currently known
reference.