In gutils/gimagereadtiff.c, input filetype is rb not b. Report read error.
In gutils/gimagereadtiff.c, exit if error reading width or height
In gutils/gimagereadtiff.c, replace galloc with malloc. More error checks.
In gutils/gimagereadtiff.c, check for fileread errors. More commenting.
In gutils/gimagereadxbm.c, report if an error opening input file
In gutils/gimagereadxbm.c, get width height, exit if error.

GImageDestroy() already debugged earlier and checks for NULL, therefore
don't need to check for NULL here now. Everything here/down closes file.
In gutils/gimagereadxbm.c, exit if error reading hotspot.
In gutils/gimagereadxbm.c, expand to search for char,short or long data
In gutils/gimagereadxbm.c, change galloc to malloc. Exit if memory error.
In gutils/gimagereadxbm.c, allow for short.

While trying this, I happened to see GIMP output short for an 8x8 graphic,
so I modified routine to allow for possibility of xbm holding short instead
of expected char values. Added possibility of expecting long value but did
not write code for it since didn't have such an *.xbm holding long values.
Merge pull request #355 from JoesCat/master

tiff and xbm updates