Well, I hear it plain as day, and after 40+ years of writing, playing, and recording bass lines, I know a flub (fumble, stumble, clam, call it what you want) when I hear one, 'cuz I've certainly made my fair share. No way that was intentional. Even if you give him the benefit of the doubt, say maybe he was doing some kind of slide or gliss from G to F, it's not musical or well-timed in the least. I vote "clam."
However, I can easily believe they left it in. It has actually happened a lot over the decades in pop music (before autotune and digital manipulation), on just about every instrument. Who knows what the prevailing mentality was, or where McVie's head was that day, but this might have been the best he was good for, and it's a relatively minor flub that doesn't wreck the track. It probably could have been easily punched in/out and fixed, but then you sometimes end up with an audible hiccup or volume discrepancy (again, pre-digital era).