static-build: fix readline read of uninit variable

This patch fixes potential read of uninitialized variable
in history_truncate_file()

Fixed in upstream:
https://git.savannah.gnu.org/cgit/readline.git/commit/?h=devel&id=b4ebdc06601fb54297435d2e286d901cba1cd6c6

Closes tarantool/security#95

NO_DOC=security
NO_TEST=security
NO_CHANGELOG=security

(cherry picked from commit 2087154a)