Fix warning about uninitialized variable

With gcc 4.2.1-sjlj (mingw32-2) I get this warning:
/src/qemu/exec.c: In function 'qemu_ram_alloc':
/src/qemu/exec.c:2777: warning: 'offset' may be used uninitialized in this function

Fix by initializing the variable.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
