bmp: Avoid allocating large buffers when not needed
bmp: Avoid allocating large buffers when not needed

Avoid allocating nearly 6 gigs of data when parsing the CVE-2015-4491
test only to free it when we've calculated that the rowstride * height
would overflow by calculating the rowstride before doing the allocation,
using the new gdk_pixbuf_calculate_rowstride() helper.

https://bugzilla.gnome.org/show_bug.cgi?id=765094

GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf,GNOME/gdk-pixbuf