mesa: change assert to unreachable in two format functions

This fixes two problems reported by osc:
I: Program returns random data in a function
E: Mesa no-return-in-nonvoid-function ../../src/mesa/main/format_utils.c:180
E: Mesa no-return-in-nonvoid-function ../../src/mesa/main/glformats.c:2714

Reviewed-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>

src/mesa/main/format_utils.c

@@ -175,7 +175,7 @@ _mesa_compute_rgba2base2rgba_component_mapping(GLenum baseFormat, uint8_t *map)
          return needRebase;
       }
    default:
-      assert(!"Unexpected base format");
+      unreachable("Unexpected base format");
    }
 }