app: Fix md5 hash calculation
app: Fix md5 hash calculation

Don't create a new instance of digest every time, it won't work.  Old
version returned md5("") for every document because of this.

Signed-off-by: Aki Saarinen <278bc57fbbff6b7b38435aea0f9d37e3d879167e@akisaarinen.fi>

akisaarinen/ccf,akisaarinen/ccf