mkrasimage: fix segmentation fault

mkrasimage: fix segmentation fault

Code was attempting to determine the size of the file
before it was actually known and allocating insufficient
memory space.  Images above a certain size caused a
segmentation fault.  Moving the calloc() ensured ensured
that large images didn't result in a buffer overflow on
memcpy().

Signed-off-by: Michael T Farnworth <michael@turf.org>
[fixed name in From to match one in SoB]
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit b468353a373d181c4362ff690d7b22a08f5f6949)