Fixed-length  buffers to SHA1 size lead to crash when SHA 256 is used

This fix isn't optimal. The siez of the arrays should be determined by
the hash algorithm. The change in create.c fixes the overwriting of
variable i, which was already found due to documentation.
There was an additional crash, when the buffers get deleted. The
checkhash member was suffering buffer overflow. This is fixed by the
change in netpgpdigest.h.