mesa: Prevent _MaxLevel from being less than zero

When decoding using VDPAU, the _MaxLevel value becomes -1 due to
NumLevels being equal to 0 at a certain point, and decoding fails
due to an assertion later on.

Signed-off-by: Thong Thai <thong.thai@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Cc: 19.2 19.3 <mesa-stable@lists.freedesktop.org>

src/mesa/main/texobj.c

@@ -757,7 +757,7 @@ _mesa_test_texobj_completeness( const struct gl_context *ctx,
       /* Adjust max level for views: the data store may have more levels than
        * the view exposes.
        */
-      t->_MaxLevel = MIN2(t->_MaxLevel, t->NumLevels - 1);
+      t->_MaxLevel = MAX2(MIN2(t->_MaxLevel, t->NumLevels - 1), 0);
    }
 
    /* Compute _MaxLambda = q - p in the spec used during mipmapping */