patch 8.2.2978: warning for uninitialized variable

Problem:    Warning for uninitialized variable.
Solution:   Set return value to FAIL.
diff --git a/src/eval.c b/src/eval.c
index 0d15a70..ca181d5 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -3777,6 +3777,7 @@
 	    if (s == NULL || *s == NUL)
 	    {
 		emsg(_(e_empty_function_name));
+		ret = FAIL;
 		goto theend;
 	    }
 	}