Place an upper bound on the number of iterations for REP.

Right now it's possible to invoke the REP escape sequence with a maximum
of tens of millions of iterations. In practice, there is never any need
to do this. Calling it more frequently than the number of cells in the
terminal hardly makes any sense. By placing a limit on it, we can
prevent users from exhausting resources in inside the terminal emulator.

As support for this escape sequence is not present in any of the stable
branches, there is no need to MFC.

Reported by: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=11255