Fixed GMimeFilter to properly alloc the outreal and backbuf buffers
Fixed GMimeFilter to properly alloc the outreal and backbuf buffers

When requesting a backup our output buffer of size 0,
the previous logic would no-op thus allowing said
buffers to remain NULL if not already allocated.

This would break the expectation that the filtered output
buffer would never be set to NULL.

jstedfast/gmime,jstedfast/gmime,GNOME/gmime,GNOME/gmime,GNOME/gmime,jstedfast/gmime