Allow writing to AVPs with negative index

This includes both SET and DELETE operations, for example:
    $(avp(foo)[-1]) = NULL;
    $(avp(foo)[-1]) = "42";

(cherry picked from commit df362fb)

pvar.c

@@ -2787,11 +2787,6 @@ int pv_set_avp(struct sip_msg* msg, pv_param_t *param,
 			destroy_avps(name_type, avp_name, 1);
 		else
 		{
-			if(idx < 0)
-			{
-				LM_ERR("Index with negative value\n");
-				return -1;
-			}
 			destroy_index_avp(name_type, avp_name, idx);
 		}
 		return 0;

usr_avp.c

@@ -201,8 +201,14 @@ int replace_avp(unsigned short flags, int name, int_str val, int index)
 	struct usr_avp* avp_new, *avp_del;
 
 	if(index < 0) {
-		LM_ERR("Index with negative value\n");
-		return -1;
+		/* convert negative index to 0+ */
+		int pidx = count_avps(flags, name) + index;
+		if (pidx < 0) {
+			LM_DBG("AVP with the specified index (%d) not found\n", index);
+			return -1;
+		}
+
+		index = pidx;
 	}
 
 	avp_del = search_index_avp(flags, name, 0, index);
@@ -415,6 +421,17 @@ void destroy_index_avp( unsigned short flags, int name, int index)
 {
 	struct usr_avp *avp = NULL;
 
+	if(index < 0) {
+		/* convert negative index to 0+ */
+		int pidx = count_avps(flags, name) + index;
+		if (pidx < 0) {
+			LM_DBG("AVP with the specified index (%d) not found\n", index);
+			return;
+		}
+
+		index = pidx;
+	}
+
 	avp = search_index_avp(flags, name, 0, index);
 	if(avp== NULL) {
 		LM_DBG("AVP with the specified index not found\n");
@@ -424,6 +441,17 @@ void destroy_index_avp( unsigned short flags, int name, int index)
 	destroy_avp( avp );
 }
 
+int count_avps(unsigned short flags, int name)
+{
+	struct usr_avp *avp = NULL;
+	int n = 0;
+
+	while ((avp=search_first_avp(flags, name, 0, avp)))
+		n++;
+
+	return n;
+}
+
 void destroy_avp_list_bulk( struct usr_avp **list )
 {
 	struct usr_avp *avp, *foo;

usr_avp.h

@@ -99,6 +99,7 @@ struct usr_avp *search_index_avp(unsigned short flags,
 
 /* free functions */
 void reset_avps( );
+int  count_avps(unsigned short flags, int name);
 void destroy_avp( struct usr_avp *avp);
 void destroy_index_avp( unsigned short flags, int name, int index);
 int  destroy_avps( unsigned short flags, int name, int all);