If mrContext is zero, the CG calls using it won't do anything and will
cause "invalid context" warnings when running under Xcode. So just
return early. Such calls to drawPolyPolygon() are apparently not
essential but accidental and of no real use. Possibly related to bogus
"focus" indicator painting. Which does not make sense on iOS as we
aren't drawing to any actual "windows" with "focus".

Change-Id: Icc4a768d8b255c09a8972551443ce59f4ad56089