patch 8.1.0388: Coverity complains about possible NULL pointer use

Problem:    Coverity complains about possible NULL pointer use.
Solution:   Use get_tv_string() instead of get_tv_string_chk().
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 69a674d..f49bcb8 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -13075,7 +13075,7 @@
 	EMSG(_(e_invarg));
     else
     {
-	name = get_tv_string_chk(&argvars[0]);
+	name = get_tv_string(&argvars[0]);
 	if (reset_option_was_set(name) == FAIL)
 	    EMSG2(_(e_invarg2), name);
     }