evolve: check stablesortcache index size before unpacking (issue6354)

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branch : stable
8 jobs for topic/stable/cache-part-2 in 9 minutes and 28 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed checks-py2 #185145

00:01:34

passed checks-py3 #185146

00:01:59

passed doc #185151

00:01:18

manual pytype #185144
allowed to fail manual
passed tests-py2-cext #185147

00:08:06

passed tests-py2-pure #185148

00:03:03

passed tests-py3-cext #185149

00:06:31

passed tests-py3-pure #185150

00:06:46