Fix double free in NanostackInterface

When freeing all memory in the rx buffer chain set the head pointer to
NULL. This prevents the head rx buffer from getting freed twice.

features/net/FEATURE_IPV6/nanostack-interface/NanostackInterface.cpp

@@ -368,6 +368,7 @@ void NanostackSocket::data_free_all(void)
     // No mode requirement
 
     NanostackBuffer *buffer = rxBufChain;
+    rxBufChain = NULL;
     while (buffer != NULL) {
         NanostackBuffer *next_buffer = buffer->next;
         FREE(buffer);