mesa: Return after assertion failure.

Addresses the warnings:
warning: ‘target’ may be used uninitialized in this function
warning: ‘target_string’ may be used uninitialized in this function

src/mesa/program/ir_to_mesa.cpp

@@ -2456,7 +2456,7 @@ get_mesa_program(GLcontext *ctx, struct gl_shader_program *shader_program,
       break;
    default:
       assert(!"should not be reached");
-      break;
+      return NULL;
    }
 
    validate_ir_tree(shader->ir);