[runtime] Fix return of uninit value (#11823)

We have a return on line 3608 that doesn't have an
obvioius assignment to the variable "o" before that.

mono/metadata/object.c

@@ -3527,7 +3527,7 @@ mono_field_get_value_object_checked (MonoDomain *domain, MonoClassField *field,
 
 	error_init (error);
 
-	MonoObject *o;
+	MonoObject *o = NULL;
 	MonoClass *klass;
 	MonoVTable *vtable = NULL;
 	gpointer v;