Memory leak in postpos.c

[alecianfe]

Hi,
as for the RTKLIB repository, there is the possibility to experience memory leak on postpos.c when function readobsnav() fails. This is due to the fact that there is no call to freeobsnav() in case of failure. I've proposed an update to the official repository, so it could be useful also for yours.

Link to the issue:
https://github.com/tomojitakasu/RTKLIB/issues/443
Link to the pull request:
tomojitakasu#444