searchidx: avoid modification of read-only `$_'

searchidx: avoid modification of read-only `$_'

This fixes the "Modification of a read-only value attempted at ..."
error in an initial run of t/reindex-time-range.t.  It was
reproducible by running `rm -rf t/data-gen/reindex-time-range.v*'
before `make && prove -bvw t/reindex-time-range.t'.  Thanks to
Jörg Rödel for providing the backtrace which helped find this.

Debugged-by: Jörg Rödel <joro@8bytes.org>
Link: https://public-inbox.org/meta/YZuZEY+WSnm4wlrS@8bytes.org/