OS_Find with a path

If you try and use OS_Find to open a file, using a path specified in R2, but specifiy the flag bits incorrectly, you get an undefined instruction error rather than something friendly.

Example: in this case bit 1 of R0 should be set, not bit 0.

SYS"OS_Find",&40 OR %1101, "Boot.Desktop", "Choices$Path" TO i% : P.i%

Internal error: undefined instruction at &FB413A10

In fact this looks suspiciously like this other bug I reported a while ago: https://www.riscosopen.org/tracker/tickets/287