Make the OSX buildbot ramdisk scripts safer.

[in tools/buildbot/slaves/svn-x64-macosx]
* mkramdisk.sh, rmramdisk.sh: Check if the volume mountpoint exists.
   This prevents unintended side effects if one builder name is
   a prefix of another.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1638766 13f79535-47bb-0310-9956-ffa450edef68