Some XPK entries have zero width and zero height

Some XPK files -- exclusively those which represent journal entry fonts, apparently -- claim to have sub-images which are of dimensions 0x0.

charlotte@dawnchorus:~/devel/lunapurpura/TRICKY/j/IDGlobal ../../../build/src/xpk/xpk  -c 44000-IDPalette.CLU -p -o 52000-Rockett18.XPK.png 52000-Rockett18.XPK 
96 entry(ies):
    XPKEntry offset=1936 dimensions=0x0 pos=0x0
    XPKEntry offset=1936 dimensions=2x15 pos=8x1
    XPKEntry offset=1951 dimensions=4x6 pos=6x0
    XPKEntry offset=1974 dimensions=13x11 pos=6x1
    XPKEntry offset=2034 dimensions=10x19 pos=6x0
    XPKEntry offset=2111 dimensions=10x17 pos=6x1
    XPKEntry offset=2198 dimensions=8x19 pos=6x0
    XPKEntry offset=2245 dimensions=2x4 pos=6x0
    XPKEntry offset=2253 dimensions=5x18 pos=6x0
    XPKEntry offset=2285 dimensions=8x20 pos=4x0
    XPKEntry offset=2323 dimensions=7x7 pos=7x2

Of course, this screws things up when you're trying to actually decode the images:

libpng warning: Image width is zero in IHDR
libpng warning: Image height is zero in IHDR
libpng error: Invalid IHDR data
xpk: couldn't write PNG!: Error

So, what to do?

Maybe the 0x0 dimensions has a certain significance -- i.e., perhaps it implies something that we should be paying attention to.
Or maybe it's just something we're supposed to ignore, and so we need to skip over it.
Or maybe there's a bug in my XPK parser.

dressupgeekout / lunapurpura

Some XPK entries have zero width and zero height #54