patch 9.1.0674: Vim9: compiling abstract method fails because of missing return

Problem:  Vim9: compiling abstract method fails because of missing
          return (Aliaksei Budavei)
Solution: don't require a return statement for an abstract method when
          compiling (Ernie Rael)

fixes: #15432
closes: #15441

Signed-off-by: Ernie Rael <errael@raelity.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/vim9compile.c b/src/vim9compile.c
index ea305b7..de13f9b 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -4120,8 +4120,9 @@
     ufunc->uf_args_visible = ufunc->uf_args.ga_len;
 
     // Compiling a function in an interface is done to get the function type.
-    // No code is actually compiled.
-    if (ufunc->uf_class != NULL && IS_INTERFACE(ufunc->uf_class))
+    // No code is actually compiled. Same goes for an abstract method.
+    if ((ufunc->uf_class != NULL && IS_INTERFACE(ufunc->uf_class))
+	|| IS_ABSTRACT_METHOD(ufunc))
     {
 	ufunc->uf_def_status = UF_NOT_COMPILED;
 	ret = OK;