i915g: Fix 'control reaches end of non-void function' warning.

Fixes the following GCC warning.
i915_screen.c: In function 'i915_get_shader_param':
i915_screen.c:184: warning: control reaches end of non-void function

src/gallium/drivers/i915/i915_screen.c

@@ -179,7 +179,8 @@ i915_get_shader_param(struct pipe_screen *screen, unsigned shader, enum pipe_sha
       case PIPE_SHADER_CAP_TGSI_CONT_SUPPORTED:
          return 0;
       default:
-              break;
+         assert(0);
+         return 0;
    }
 }