mesa: Prevent repeated glDeleteProgram() from blowing away our refcounts.

glDeleteProgram should only be able to remove the one refcount for the
user's reference to the program from the hash table (even though that
ref does live on in the hash table until the last other ref is
removed).

Fixes piglit ARB_shader_objects/delete-repeat.

Reviewed-by: Chad Versace <chad@chad-versace.us>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/main/shaderapi.c

@@ -390,10 +390,12 @@ delete_shader_program(struct gl_context *ctx, GLuint name)
    if (!shProg)
       return;
 
-   shProg->DeletePending = GL_TRUE;
+   if (!shProg->DeletePending) {
+      shProg->DeletePending = GL_TRUE;
 
-   /* effectively, decr shProg's refcount */
-   _mesa_reference_shader_program(ctx, &shProg, NULL);
+      /* effectively, decr shProg's refcount */
+      _mesa_reference_shader_program(ctx, &shProg, NULL);
+   }
 }