DSASigner: fix key check when no digest specified

Since the digest specified was a NullDigest, its effective length during
initialization is 0 bytes, so there is no way to tell if the key
strength is appropriate at that time.

Bug: 21630204
Bug: 27247313
Change-Id: I0f60ac5d62ae54119fd432cf828a2282418e8f39
1 file changed