Ferrer is 3-0 against Murray on clay, also this year is Murray's worst claycourt season since 4 years ago.
But they never played at Roland Garros. In a best-of-five Grand Slam match, one seriously has to fancy Murray, even if it's on clay, assuming that he is healthy enough to get to the QF in the first place.
It'd be quite bad for Murray to lose a Slam match to a player that is, in many ways, an inferior version of Murray himself. That said, if Murray does crash and burn it's hard to see anyone stop Ferrer before SFs.