patch 8.1.0635: Coverity complains about null pointer use

Problem:    Coverity complains about null pointer use.
Solution:   Avoid using a null pointer.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 99e68f7..a9ef60e 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -13586,7 +13586,7 @@
 	EMSG(_(e_invarg));
     else
     {
-	name = tv_get_string_chk(&argvars[0]);
+	name = tv_get_string(&argvars[0]);
 	val = (int)tv_get_number(&argvars[1]);
 
 	if (STRCMP(name, (char_u *)"redraw") == 0)