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 <[email protected]>
[fixed name in From to match one in SoB]
Signed-off-by: Petr Štetiar <[email protected]>