mesa: Fix a crash in update_texture_state() for external texture type

NOTE: This is a candidate for the stable branch.

Signed-off-by: Abdiel <abdiel.janulgue@intel.com>
Signed-off-by: Brian Paul <brianp@vmware.com>

src/mesa/main/texobj.c

@@ -769,6 +769,9 @@ _mesa_get_fallback_texture(struct gl_context *ctx, gl_texture_index tex)
          target = GL_TEXTURE_BUFFER;
          break;
       case TEXTURE_EXTERNAL_INDEX:
+         dims = 2;
+         target = GL_TEXTURE_EXTERNAL_OES;
+         break;
       default:
          /* no-op */
          return NULL;

src/mesa/main/texstate.c

@@ -600,6 +600,13 @@ update_texture_state( struct gl_context *ctx )
 
             texTarget = (gl_texture_index) (ffs(enabledTargets) - 1);
             texObj = _mesa_get_fallback_texture(ctx, texTarget);
+            
+            assert(texObj);
+            if (!texObj) {
+               /* invalid fallback texture: don't enable the texture unit */
+               continue;
+            }
+
             _mesa_reference_texobj(&texUnit->_Current, texObj);
             texUnit->_ReallyEnabled = 1 << texTarget;
          }