[ReflectionProbes] ReflectionProbe cubemap causes assertion on re-import

Changing colorspace causes assert in the console

1. Open project 'trunk 2017.2'
2. Open scene 's1'
3. In Player Settings, switch colorspace
4. Observe assertion is thrown in the console

alternatively, these steps produce same assert:

1. In Project view, select ReflectionProbe-0 cubemap
2. In Inspector, change Convolution Type from Specular to Irradiance

Need fix for 2017.2

Already fixed in 2017.1