This panic could occur when samples with too long label values are ingested into VictoriaMetrics.
This could result in too long fistItem and commonPrefix values at blockHeader (up to 64kb each).
This may inflate the maximum index block size by 4 * maxIndexBlockSize.
ioutil.ReadAll is deprecated since Go1.16 - see https://tip.golang.org/doc/go1.16#ioutil
VictoriaMetrics requires at least Go1.18, so it is OK to switch from ioutil.ReadAll to io.ReadAll.
This is a follow-up for 02ca2342ab