st/mesa: Initialize textures array in st_framebuffer_validate

st/mesa: Initialize textures array in st_framebuffer_validate

And just reference pipe_resources to it in the validate callbacks.

Avoids pipe_resource leaks when st_framebuffer_validate ends up calling
the validate callback multiple times, e.g. when a window is resized.

v2:
* Use generic stable tag instead of Fixes: tag, since the problem could
  already happen before the commit referenced in v1 (Thomas Hellstrom)
* Use memset to initialize the array on the stack instead of allocating
  the array with os_calloc.

Cc: mesa-stable@lists.freedesktop.org
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>