Fix crash when calling script functions with omitted optional params

The crash would happen if the script function is exported with the
CMD_PARAM_FIX_NULL flag and does not actually change the parameter
value. Currently, only forward() uses the flag in this manner and
is the only function affected by this crash.

(cherry picked from commit 278c164)

Author: rvlad-patrascu

mod_fix.c

@@ -162,21 +162,20 @@ int fix_cmd(struct cmd_param *params, action_elem_t *elems)
 			gp = NULL;
 			if (param->flags & CMD_PARAM_OPT) {
 				if (param->fixup && (param->flags & CMD_PARAM_FIX_NULL)) {
-					if ((gp = alloc_gp()) == NULL)
-						return E_OUT_OF_MEM;
-
-					gp->pval = NULL;
-					gp->type = GPARAM_TYPE_VAL;
-
-					h = gp->pval;
-					if (param->fixup(&gp->pval) < 0) {
+					h = NULL;
+					if (param->fixup(&h) < 0) {
 						LM_ERR("Fixup failed for param [%d]\n", i);
 						ret = E_UNSPEC;
 						goto error;
 					}
-					if (h!=gp->pval)
-						gp->type = GPARAM_TYPE_FIXUP;
 
+					if (h != NULL) {
+						if ((gp = alloc_gp()) == NULL)
+							return E_OUT_OF_MEM;
+
+						gp->type = GPARAM_TYPE_FIXUP;
+						gp->pval = h;
+					}
 				}
 
 				goto fill_elems;