dialog: Mitigate 'dlg_list' PKG fragmentation

By using pkg_str_extend() instead of pkg_realloc(), we avoid a constant
creation of PKG fragments (the buffer will eventually stop growing),
which seems to gradually fragment the private memory pool, as the MI
'dlg_list' is continuously polled by various monitoring software.

Related to #3235

(cherry picked from commit d2037f6)

modules/dialog/dlg_hash.c

@@ -1325,7 +1325,7 @@ void next_state_dlg(struct dlg_cell *dlg, int event, int dir, int *old_state,
 
 
 /**************************** MI functions ******************************/
-static char *dlg_val_buf;
+static str dlg_val_buf;
 static inline int internal_mi_print_dlg(mi_item_t *dialog_obj,
 									struct dlg_cell *dlg, int with_context)
 {
@@ -1477,11 +1477,12 @@ static inline int internal_mi_print_dlg(mi_item_t *dialog_obj,
 			/* print dlg values -> iterate the list */
 			for( dv=dlg->vals ; dv ; dv=dv->next) {
 				/* escape non-printable chars */
-				p = pkg_realloc(dlg_val_buf, 4 * dv->val.len + 1);
-				if (!p) {
-					LM_ERR("not enough mem to allocate: %d\n", dv->val.len);
+				if (!pkg_str_extend(&dlg_val_buf, 4 * dv->val.len + 1)) {
+					LM_ERR("not enough mem to allocate: %d\n", 4 * dv->val.len + 1);
 					continue;
 				}
+
+				p = dlg_val_buf.s;
 				for (i = 0, j = 0; i < dv->val.len; i++) {
 					if (dv->val.s[i] < 0x20 || dv->val.s[i] >= 0x7F) {
 						p[j++] = '\\';
@@ -1507,8 +1508,6 @@ static inline int internal_mi_print_dlg(mi_item_t *dialog_obj,
 					goto error;
 				if (add_mi_string(values_item,dv->name.s,dv->name.len,p,j) < 0)
 					goto error;
-
-				dlg_val_buf = p;
 			}
 		}