Validate buffer length before reading fields in Packet::readFrom

readFrom reads the header byte, transport codes (4 bytes), and
path_len from the source buffer before any length validation. With a
short input, these reads go past the end of the buffer.

Add upfront length checks: minimum 2 bytes overall, transport codes
require 4 additional bytes, and path must fit before the remaining
payload.

Author: weebl2000

src/Packet.cpp

@@ -39,18 +39,20 @@ uint8_t Packet::writeTo(uint8_t dest[]) const {
 }
 
 bool Packet::readFrom(const uint8_t src[], uint8_t len) {
+  if (len < 2) return false;  // minimum: header + path_len
   uint8_t i = 0;
   header = src[i++];
   if (hasTransportCodes()) {
+    if (i + 4 >= len) return false;  // need 4 bytes for transport codes + path_len after
     memcpy(&transport_codes[0], &src[i], 2); i += 2;
     memcpy(&transport_codes[1], &src[i], 2); i += 2;
   } else {
     transport_codes[0] = transport_codes[1] = 0;
   }
   path_len = src[i++];
   if (path_len > sizeof(path)) return false;   // bad encoding
+  if (i + path_len >= len) return false;  // path + at least 1 byte payload must fit
   memcpy(path, &src[i], path_len); i += path_len;
-  if (i >= len) return false;   // bad encoding
   payload_len = len - i;
   if (payload_len > sizeof(payload)) return false;  // bad encoding
   memcpy(payload, &src[i], payload_len); //i += payload_len;