glsl/nir: init non-static class member.

glsl_to_nir.cpp:276: uninit_member: Non-static class member "sig" is not initialized in this constructor nor in any functions that it calls.

Reported by coverity

Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>

src/compiler/glsl/glsl_to_nir.cpp

@@ -270,6 +270,7 @@ nir_visitor::nir_visitor(gl_context *ctx, nir_shader *shader)
    this->result = NULL;
    this->impl = NULL;
    this->deref = NULL;
+   this->sig = NULL;
    memset(&this->b, 0, sizeof(this->b));
 }