Fix reading pascal string

[gyk]

I encountered a PSD file that caused psd crate to panic. The problems are:

• read_pascal_string seems to have a bug. Pascal string is padded to make the size even, so the extra byte only exists when data length + 1 (header byte) is odd. Related to #40. Ref implementation: https://github.com/ImageMagick/ImageMagick/blob/7.1.1-3/coders/psd.c#L798-L799
• It contains non-UTF-8 strings.
• The last channel (blue) has a trailing zero byte. Not sure whether it is unnecessary padding, but otherwise the image data itself is intact. It would be nice to make the crate more tolerant of possibly malformed files.

This PR attempts to resolve above issues.

Update: Also fixes panicking when the Layer and mask information section is empty (Krita can produce such PSDs).

[chinedufn]

Thanks!

We'll need a test case that has a minimal PSD file where

• Before this commit, we could not read back some string
• As of this commit, we're able to read back the string

The test can live somewhere in here https://github.com/chinedufn/psd/tree/master/tests

After that we can get this merged.

[gyk]

I've added a 1x1 PSD that

• Contains a non-UTF-8 Pascal string of 10 bytes in an image resource block, which is directly copied from a real world PSD file.
• Has zero-length layer and mask information section.

[chinedufn]

Thanks for getting this all tested!