I have reproduced this both on my Gentoo installation and on an Archlinux installation. But thinking of the rationale of the patch, you are right, "is_muting" is unnecessary, what I am not sure of though, is the original

mii->mute ? "" : " [muted]");

as surely, it is the other way around, i.e. if mute is true, it outputs nothing, otherwise, if mute is false (well, 0 as it is an int) it outputs "muted".

I think the reason this is this way around, is that in "void pulseaudio_toggle_mute(menu_info_item_t* mii)" the value of mii->muted is not changed, only the local variable "int mute" is.