[2979] Update the current packet pointer when removing timed out packet.

perfdhcp tracks sent packets. If it finds timed out packet it is removed
from the list of sent packets. This change updates the internal pointer
tracking next packet for which response from a server is expected. When
this is not done, it may lead to use of pointer to the removed element
and crash.

tests/tools/perfdhcp/stats_mgr.h

@@ -455,9 +455,16 @@ public:
                             packet_period.length().total_seconds() +
                             (static_cast<double>(packet_period.length().fractional_seconds())
                              / packet_period.length().ticks_per_second());
-                        if (drop_time_ > 0 &&
-                            (period_fractional > drop_time_)) {
-                            eraseSent(sent_packets_.template project<0>(it));
+                        if (drop_time_ > 0 && (period_fractional > drop_time_)) {
+                            // The packet pointed to by 'it' is timed out so we
+                            // have to remove it. Removal may invalidate the
+                            // next_sent_ pointer if it points to the packet
+                            // being removed. So, we set the next_sent_ to point
+                            // to the next packet after removed one. This
+                            // pointer will be further updated in the following
+                            // iterations, if the subsequent packets are also
+                            // timed out.
+                            next_sent_ = eraseSent(sent_packets_.template project<0>(it));
                             ++collected_;
                         }
                     }