For an LVALUE fetch, "hv_fetch()" will recurse into "hv_store()" for a

For an LVALUE fetch, "hv_fetch()" will recurse into "hv_store()" for a
hash with magic. Field hashes have u magic, so this recursion triggers.
However, key conversion replaces the original key with the converted
key, so we need to ensure that conversion happens exactly once, else
for a non-idempotent key conversion routine (eg ROT13) we will see
double conversion in this case.

p4raw-id: //depot/perl@31898