Fixes the following warnings from Coverity:
* file-paa.c: Casts "width" to guint32 to prevent overflow.
Also, fix checks so that RGBA_8888 textures will be read.
* file-png.c: Free variables if APNG is not fully loaded
* file-seattle-filmworks.c: Verify fseek () doesn't exceed
length of file
* file-tim.c: Casts "width" to guint32 to prevent overflow
* guillotine.c: Free "file" if image creation fails
* gimpitem.c: Initialize offset_x/y to 0 to remove warning
aa9aa9e489