CVE-2026-4046: fix assertion failure in IBM1390 and IBM1399 iconv modules
(RHEL-162891). Minimal backport of the iconvdata/ibm1364.c hunks from
upstream commit d6f08d1cf027 (Red Hat glibc-2.28-251.el8_10.37,
RHSA-2026:20587) — uses the __count state variable to store the pending
4-byte UCS-4 code point so conversion can resume across output-buffer
boundaries.