BUILD: quic: remove GCC undefined error in qc_release_lost_pkts()

BUILD: quic: remove GCC undefined error in qc_release_lost_pkts()

Every once in a while, GCC reports issues with qc_release_lost_pkts()
function. It seems that its static analysis is foiled by the code
structuring. The latest warning reports the following issue :

  CC      src/quic_loss.o
src/quic_loss.c: In function ‘qc_release_lost_pkts’:
src/quic_loss.c:313:58: error: potential null pointer dereference [-Werror=null-dereference]
  313 |                         unsigned int period = newest_lost->time_sent_ms - oldest_lost->time_sent_ms;
      |                                               ~~~~~~~~~~~^~~~~~~~~~~~~~

To fix definitely this, change slightly the code. <oldest_lost> and
<newest_lost> are now initialized on the first list entry outside of the
loop. This is enough to guarantee to GCC that they cannot be NULL for
the remainder of the function.