Steps to repro:

1. Open attached project and 'test' scene;
2. In the upper context menu bar select EXR -> Save RenderTexture;
3. Drag 'New Render texture' file from the Assets folder to an Input field of the opened window;
4. Click 'Apply' button several time (more that 6).

Actual result:
After several clicks Editor will crash. See attached gif video.

Last calls from the stacktrace:

0x00000001409E7FE0 (Unity) details::IdentityFunc::CopyImage<details::Rgba32FloatToRgba32FloatConverter>
0x00000001409EC8CB (Unity) details::CopyImageImpl<details::IdentityFunc>
0x00000001409F06CB (Unity) GfxDeviceD3D11Base::ReadbackImage
0x0000000140A5BC95 (Unity) GfxDeviceWorker::RunCommand
0x0000000140A602DF (Unity) GfxDeviceWorker::Run
0x0000000140A43E7D (Unity) GfxDeviceWorker::RunGfxDeviceWorker
0x0000000141013D7C (Unity) Thread::RunThreadWrapper
0x00007FFAC7D78364 (KERNEL32) BaseThreadInitThunk
0x00007FFAC7F35E91 (ntdll) RtlUserThreadStart

Expected result:
Editor should not crash when converting textures.

Notes:
- Possible reason of this bug is that there is no proper code path for converting from RGBA32 RenderTexture to float32 texture when using ReadPixels API;
- Reproducible on Windows 10;
- Not reproducible in OSX 10.12.1;
- Reproducible in 5.6.0a3, 5.6.0a2;
- Wasn't able to test in 5.5, since converting to EXR feature was only introduced in 5.6, however it might also happen in earlier versions when converting to .jpg or to .png with using ReadPixels API;
- Crash may happen randomly on other graphic APIs and the invalid memory access pattern differs between platforms.