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 #185145
checks-py2

00:01:34

passed #185146
checks-py3

00:01:59

passed #185151
doc

00:01:18

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

00:08:06

passed #185148
tests-py2-pure

00:03:03

passed #185149
tests-py3-cext

00:06:31

passed #185150
tests-py3-pure

00:06:46