Removed access to potentially invalid positions on vector.

src/ip_reassembler.cpp

@@ -77,7 +77,7 @@ PDU *IPv4Stream::allocate_pdu() const {
     }
     return Internals::pdu_from_flag(
         static_cast<Constants::IP::e>(transport_proto),
-        &buffer[0],
+        buffer.empty() ? 0 : &buffer[0],
         buffer.size()
     );
 }