glx: fix length of GLXGetFBConfigsSGIX
glx: fix length of GLXGetFBConfigsSGIX

The extra length is the size of the request *minus* the size of the
VendorPrivate header, not the addition.

NOTE: This is a candidate for the 7.9 and 7.10 branches

Signed-off-by: Julien Cristau <82c7ebd12259428dfee9dc86e69f3a9d6f3dccb9@debian.org>
Signed-off-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

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