rallocx path: only set errno on the realloc case.

PR: 291677
Obtained from: jemalloc (commit 83b075789b4239035931c1ee212576d00153bbf0)
Fixes: c43cad871720 ("jemalloc: Merge from jemalloc 5.3.0 vendor branch")
MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/2059