mesa: Fix extra memset in store_tfeedback_info()

mesa: Fix extra memset in store_tfeedback_info()

Commit 9d36c96d6ec9f2c05c8e0b9ef18c5462cddee8c1 (mesa: Fix
glGetTransformFeedbackVarying()) accidentally added an extra memset()
call to the store_tfeedback_info() function, causing
prog->LinkedTransformFeedback.NumBuffers to be erased.

This patch removes the extra memset and rearranges the other
operations in store_tfeedback_info() to be in the correct order.

Fixes piglit tests "EXT_transform_feedback/api-errors *unbound*"

Reviewed-by: Eric Anholt <eric@anholt.net>