patch 9.0.0377: argument assignment does not work

Problem:    Argument assignment does not work.
Solution:   Skip over "=".
diff --git a/src/userfunc.c b/src/userfunc.c
index 0c6f993..a227031 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -311,7 +311,7 @@
 
 		// find the end of the expression (doesn't evaluate it)
 		any_default = TRUE;
-		p = skipwhite(p) + 1;
+		p = skipwhite(np + 1);
 		expr = p;
 		if (eval1(&p, &rettv, NULL) != FAIL)
 		{