BUILD: quic: Compilation fixes for some gcc warnings with -O1

BUILD: quic: Compilation fixes for some gcc warnings with -O1

This is to prevent the build from these gcc warning when compiling with -O1 option:

  willy@wtap:haproxy$ sh make-native-quic-memstat src/quic_conn.o CPU_CFLAGS="-O1"
    CC      src/quic_conn.o
  src/quic_conn.c: In function 'qc_prep_pkts':
  src/quic_conn.c:3700:44: warning: potential null pointer dereference [-Wnull-dereference]
   3700 |                                 frms = &qel->pktns->tx.frms;
        |                                         ~~~^~~~~~~
  src/quic_conn.c:3626:33: warning: 'end' may be used uninitialized in this function [-Wmaybe-uninitialized]
   3626 |                         if (end - pos < QUIC_INITIAL_PACKET_MINLEN) {
        |                             ~~~~^~~~~