cgen: fix defer with function variables (fix #12854) (#12866)

vlib/v/gen/c/fn.v

@@ -345,7 +345,9 @@ fn (mut g Gen) gen_fn_decl(node &ast.FnDecl, skip bool) {
 						}
 					}
 					info := var.obj as ast.Var
-					g.writeln('${g.typ(info.typ)}$deref $var.name;')
+					if g.table.get_type_symbol(info.typ).kind != .function {
+						g.writeln('${g.typ(info.typ)}$deref $var.name;')
+					}
 				}
 			}
 		}

vlib/v/tests/defer/defer_with_fn_var_test.v

@@ -0,0 +1,37 @@
+[has_globals]
+module main
+
+__global fcall_count = int(0)
+
+fn f1() {
+	println(1)
+	fcall_count++
+}
+
+fn f2() {
+	println(2)
+	fcall_count++
+}
+
+fn f3(f fn ()) {
+	f()
+}
+
+fn func_defer() {
+	mut func := f1
+	println('Before')
+	defer {
+		func()
+	}
+	defer {
+		f3(func)
+	}
+	func = f2
+	println('After')
+	assert true
+}
+
+fn test_defer_with_fn_var() {
+	func_defer()
+	assert fcall_count == 2
+}