Fix T46306: Cursor sometimes jumps with num-slider buttons & Continuous Grab

More precise description of the issue: Clicking on the right side of a num-slider button with Continuous Grab enabled sometimes caused cursor to jump to the inner value indicator even though text editing was started.

Happened because data->ungrab_mval wasn't reset correctly after dragging.

source/blender/editors/interface/interface_handlers.c

@@ -4529,6 +4529,10 @@ static int ui_do_but_SLI(bContext *C, uiBlock *block, uiBut *but, uiHandleButton
 				}
 			}
 			else {
+#ifdef USE_CONT_MOUSE_CORRECT
+				/* reset! */
+				copy_v2_fl(data->ungrab_mval, FLT_MAX);
+#endif
 				click = 1;
 			}
 		}