Raise ValueError if P;2L or P;4L data is truncated in frombytes()

[radarhere]

When passing data to frombytes() (or frombuffer() or im.frombytes()), it is possible to pass an insufficient amount of data. These unpackers are unusual in that they require data in multiples of 4 or 8 bytes respectively.

This is described at https://web.archive.org/web/20100206055706/http://www.qzx.com/pc-gpe/pcx.txt, where NPlanes is the 2 or 4 in P;2L or P;4L

Then calculate how many bytes are required to hold one complete uncompressed scan line:

TotalBytes = NPlanes * BytesPerLine

Note that since there are always an even number of bytes per scan line,

This PR raises a ValueError if the data is truncated.