hs-v3: Descriptor signature parsing should have a trailing white-space

From ticket legacy/trac#25552 (moved), nickm's comment:

I've reviewed the PR. The biggest issue is related to the use of "\n" signature_str, which I believe should be "\n" signature_str " " instead.

We'll fix it in legacy/trac#25552 (moved) for 034 but we need to backport it back to 030 when it was first introduced. It won't affect any tor version on the ability to parse service descriptors. It will just reinforce the security and correctness.