sdl: Fix heap smash in sdl_zoom_rgb{16,32} for int > 32 bits

Careless use of malloc(): allocate Uint32[N], assign to int *, use
int[N].

Fix by converting to g_new().

Functions can't fail anymore, so make them return void.  Caller
ignored the value anyway.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>