Fixed comparison warnings

[radarhere]

Running pip install -v . on my macOS machine, I see

  src/libImaging/TiffDecode.c:654:51: warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
      if (state->xsize != img_width || state->ysize != img_height) {
                                       ~~~~~~~~~~~~ ^  ~~~~~~~~~~
  src/libImaging/TiffDecode.c:654:22: warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
      if (state->xsize != img_width || state->ysize != img_height) {
          ~~~~~~~~~~~~ ^  ~~~~~~~~~
  2 warnings generated.

This PR fixes those warnings.