Fix function that checks validity of handshake packets

When Initial and 0-RTT packets are coalesced, they can be 1200
*together*.  lsquic_is_valid_hs_packet() should use the datagram

include/lsquic.h

@@ -1675,7 +1675,8 @@ lsquic_get_h3_alpns (unsigned versions);
  * been established: it will return incorrect result.
  */
 int
-lsquic_is_valid_hs_packet (lsquic_engine_t *, const unsigned char *, size_t);
+lsquic_is_valid_hs_packet (lsquic_engine_t *, const unsigned char *,
+                                        size_t bufsz, size_t packet_in_sz);
 
 /**
  * Parse cid from packet stored in `buf' and store it to `cid'.  Returns 0

src/liblsquic/lsquic_hspack_valid.c

@@ -88,7 +88,7 @@ is_valid_gquic_hs_packet (const unsigned char *buf, size_t bufsz,
 
 int
 lsquic_is_valid_hs_packet (struct lsquic_engine *engine,
-                                    const unsigned char *buf, size_t bufsz)
+                const unsigned char *buf, size_t bufsz, size_t packet_in_sz)
 {
     lsquic_ver_tag_t tag;
     int is_valid;
@@ -104,7 +104,7 @@ lsquic_is_valid_hs_packet (struct lsquic_engine *engine,
     case 0x80|0x00|0x20|0x10|0x08:
     case 0x80|0x40|0x20|0x10|0x00:
     case 0x80|0x00|0x20|0x10|0x00:
-        is_valid = bufsz >= IQUIC_MIN_INIT_PACKET_SZ
+        is_valid = packet_in_sz >= IQUIC_MIN_INIT_PACKET_SZ
             && lsquic_is_valid_iquic_hs_packet(buf, bufsz, &tag);
         break;
     /* 1X00 XGGG: ID-22 long header */
@@ -122,7 +122,7 @@ lsquic_is_valid_hs_packet (struct lsquic_engine *engine,
     case 0x80|0x00|0x20|0x00|0x08:
     case 0x80|0x40|0x20|0x00|0x00:
     case 0x80|0x00|0x20|0x00|0x00:
-        is_valid = bufsz >= IQUIC_MIN_INIT_PACKET_SZ
+        is_valid = packet_in_sz >= IQUIC_MIN_INIT_PACKET_SZ
             && lsquic_is_valid_ietf_v1_or_Q046plus_hs_packet(buf, bufsz, &tag);
         break;
     /* 01XX XGGG: ID-22 short header */