mesa: do not leak ctx->Shader.ReferencedProgram references

When glUseProgram is used, references to the included shaders are
added in ctx->Shader.ReferencedProgram. But those references are not
decreased when the shader data is deallocated. Thus, those shaders
are leaked.

Explicitely remove the pending references to these shaders.

Fixes: e6506b3cd2 ("mesa: retain gl_shader_programs after glDeleteProgram if they are in use")
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/mesa/main/shaderapi.c

@@ -159,6 +159,9 @@ _mesa_free_shader_state(struct gl_context *ctx)
 {
    for (int i = 0; i < MESA_SHADER_STAGES; i++) {
       _mesa_reference_program(ctx, &ctx->Shader.CurrentProgram[i], NULL);
+      _mesa_reference_shader_program(ctx,
+                                     &ctx->Shader.ReferencedPrograms[i],
+                                     NULL);
    }
    _mesa_reference_shader_program(ctx, &ctx->Shader.ActiveProgram, NULL);