* ext/stringio/stringio.c (strio_seek): Avoid signed integer overflow.
It's not harmful in practice here, but is still undefined behavior.
* ext/stringio/stringio.c (strio_extend): Check that the new length does
not exceed LONG_MAX. This fixes the invalid write on the overflow.
* test/stringio/test_stringio.rb (test_write_integer_overflow): Add a
test case for the above fix in strio_extend().
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56253 b2dd03c8-39d4-4d8f-98ff-823fe69b080e