mesa: add bounds checking to eliminate buffer overrun

mesa: add bounds checking to eliminate buffer overrun

Decompressing ETC2 textures was causing intermitent segfault
by copying resulting 4x4 texel block to the destination texture
regardless of the size of the destination texture. Issue found
via application crash in GLBenchmark 3.0's Manhattan test.

v2: add more detail comment. Compute limit outside inner loops.
v3: add bugzilla reference
v4: Correct cc syntax in commit log
v5: really grab the right patch

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74988
Cc: "9.2 10.0 10.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com> [v1, suggested v2-3]