SurfaceFlinger: check for nullptr on setFrameRate

Make sure that the layer still exists before trying to set
the frame rate for it.

Bug: 170027345
Change-Id: Ice1edd7c5320f4d1d915643e30430919de258dbc
Test: manual
(cherry picked from commit 162f643dded48b230888139d1e4c8a5e2013a7cd)
1 file changed