[perl #111000] Let hv_store work on hint hashes

[perl #111000] Let hv_store work on hint hashes

Magic attached to hash elements has its key stored differently depend-
ing on how it was supplied to hv_common.  hv_store passes a string/
length pair to hv_common, while hv_store_ent passes an SV.

magic_clearhint wasn’t able to handle string/length pairs, and only
worked with SVs, resulting in assertion failures or crashes.

This commit fixes magic_clearhint, so that XS code can use hv_store on
hint hashes.