BUG: io.save.combined() crashes with single-channel multi-plane data (meanImg_chan2 not defined)

[ytsimon2004]

Describe the issue:

When running Suite2p with single-channel input (nchannels: 1) and multiple planes (nplanes > 1) with "combined": true, the pipeline crashes during the combine step because meanImg_chan2 is referenced before assignment

Reproduce the code example:

suite2p.run_s2p(ops={
  "nchannels": 1,
  "nplanes": 4,
  "combined": true,
  ...
})

Error message:

File ".../site-packages/suite2p/io/save.py", line 181, in combined
    print(meanImg_chan2.shape)
UnboundLocalError: local variable 'meanImg_chan2' referenced before assignment

Version information:

0.14.6

[ablot]

We ran into the same issue and #1195 is enough to fix the bug.