assertion failure / segfault, format "%B %b"

[Tait <gnu...@t4...>]

Someone pointed out the other day, that:

	set format "%B %b"
	plot x

... triggers either an assertion failure and/or a segfault on 4.6p0
and 4.6p1.

I get neither, but the resulting plot is clearly broken; the ticks are
seemingly random combinations of letters and numbers that seem to bear
no relation to the plotted values.