Fix #114240: Initialize output alpha for RGB color

Pull Request: https://projects.blender.org/blender/blender/pulls/114270

source/blender/nodes/shader/nodes/node_shader_curves.cc

@@ -257,6 +257,7 @@ class CurveRGBFunction : public mf::MultiFunction {
       if (fac[i] != 1.0f) {
         interp_v3_v3v3(col_out[i], col_in[i], col_out[i], fac[i]);
       }
+      col_out[i].a = 1.0f;
     });
   }
 };