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

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

Fix this GCC warning on non-debug builds.
glsl_types.cpp: In member function 'gl_texture_index
glsl_type::sampler_index() const':
glsl_types.cpp:157: warning: control reaches end of non-void function

NOTE: This is a candidate for the 8.0 branch.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>