patch 8.2.3847: illegal memory access when using a lambda with an error

Problem:    Illegal memory access when using a lambda with an error.
Solution:   Avoid skipping over the NUL after a string.
diff --git a/src/eval.c b/src/eval.c
index f45cd8e..ecc242d 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3871,12 +3871,15 @@
 	++*arg;
 	ret = eval1(arg, rettv, evalarg);
 	*arg = skipwhite_and_linebreak(*arg, evalarg);
-	if (**arg != ')')
+	if (**arg == ')')
+	{
+	    ++*arg;
+	}
+	else
 	{
 	    emsg(_(e_missing_closing_paren));
 	    ret = FAIL;
 	}
-	++*arg;
     }
     if (ret != OK)
 	return FAIL;