This affected sculpt, vertex- and weightpaint.
So attempting to (temporarily) switching to the smooth/blur tools from
another tool using the "Shift" shortcut can fail if the corresponding
smooth/blur brush is not found/missing [which was the case in the report
because the brush was deleted].
In this case, brushes dont really get switched, but blender would still
try to cache the size (because the smooth/blur brush temporarily uses
the same size as the previous brush) of the smooth brush in
`StrokeCache` (see `smooth_brush_toggle_on`). Then in
`smooth_brush_toggle_off` it was assumed brushes were actually switched
and the (non-existing) size of the (missing) smooth brush was applied to
the **actual** brush.
Now restructure code a bit so in the case of a missing brush we can
early out (without affecting the **actual** brush then).
