BSpinner: restore local copy of control value

8cc8ec9ef9 fixed a problem due to BSpinner
using its own variable to store the value instead of resorting to
BControl's one. That was accessed with a inline method, so programs
compiled before the change get the spinner's value from a place that in
newer revisions contain garbage. Copy the value to that position after
changes, so that it is available for both old and new apps.

Fixes: #18898
Change-Id: I01679bfcb5b208e04ea85d7ec38cc655305d532a
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7657
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
Haiku-Format: Haiku-format Bot <no-reply+haikuformatbot@haiku-os.org>

src/kits/interface/Spinner.cpp

@@ -241,6 +241,8 @@ BSpinner::SetValue(int32 value)
 		return;
 
 	BControl::SetValue(value);
+	((int32*)_reserved)[0] = Value();
+
 	ValueChanged();
 
 	Invoke();