glx/tests: Fix off-by-one error in allocating extension string buffer
glx/tests: Fix off-by-one error in allocating extension string buffer

NOTE: This is a candidate for the 8.0 release branch.

Signed-off-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=50621
Bugzilla: https://bugs.gentoo.org/show_bug.cgi?id=418161
Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>
Cc: Markus Oehme <6294fbc9952df39e9bc1ec01274f585a63539290@gmx.de>

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