mesa: fix out of bounds access in glGetFramebufferParameterivEXT

ColorDrawBuffer is an array of MAX_DRAW_BUFFERS == 8.

Found by Coverity.
Signed-off-by: Marcin Ślusarz <marcin.slusarz@intel.com>
Fixes: 7534c536

 ("mesa: add EXT_dsa (Named)Framebuffer functions")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <!6067>
(cherry picked from commit 0906d5d5)

.pick_status.json

@@ -220,7 +220,7 @@
         "description": "mesa: fix out of bounds access in glGetFramebufferParameterivEXT",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "master_sha": null,
         "because_sha": "7534c536ca0f4b2b123200f421460094034f37a3"
     },

src/mesa/main/fbobject.c

@@ -4814,7 +4814,11 @@ _mesa_GetFramebufferParameterivEXT(GLuint framebuffer, GLenum pname,
          *param = fb->ColorReadBuffer;
       }
       else if (GL_DRAW_BUFFER0 <= pname && pname <= GL_DRAW_BUFFER15) {
-         *param = fb->ColorDrawBuffer[pname - GL_DRAW_BUFFER0];
+         unsigned buffer = pname - GL_DRAW_BUFFER0;
+         if (buffer < ARRAY_SIZE(fb->ColorDrawBuffer))
+            *param = fb->ColorDrawBuffer[buffer];
+         else
+            _mesa_error(ctx, GL_INVALID_ENUM, "glGetFramebufferParameterivEXT(pname)");
       }
       else {
          _mesa_error(ctx, GL_INVALID_ENUM, "glGetFramebufferParameterivEXT(pname)");