Description:
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If a script accesses multiple repositories, a segmentation fault may occur after the script finishes.  This is caused by SVN "pools" not being destroyed safely when svn-repos resources are destructed.

When an svn-repos resource is created using svn_repos_open(), a subpool is allocated to this resource.  This subpool belongs to a root pool (SVN_G(pool)), initialized earlier by the extension.  The destructor for svn-repos destroys the resource's subpool.  The RSHUTDOWN function destroys the root pool, which also destroys all svn-repos subpools.

When a script finishes, the extension's RSHUTDOWN function is called before the svn-repos destructors are called, so the subpools are destroyed twice (assuming the resources weren't destructed beforehand), which may cause a segfault.  It seems the more svn-repos resources there are alive, the more likely there will be a segfault.

Reproduce code:
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Reproducible with this script:  http://pastebin.com/xuP7YUnL
- Create 100 repositories (repo0, repo1, ..., repo99) in the same folder as this script
- Not reproducible through CLI -- only through Apache
- May produce segfaults in Apache's error.log (run the script several times)

Patch: http://pastebin.com/d5cWURH0
- This adds a check to ensure the root pool still exists before destroying an svn-repos resource's subpool

Expected result:
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Script runs normally without any segfaults.

Actual result:
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Script does one of the following:
- Runs normally without any segfaults
- Runs normally, but a segfault is logged in Apache's error.log
- Crashes Apache, requiring restarting Apache
- Crashes with a segfault and no output to browser