Fix bs_skip_bits2
Fix bs_skip_bits2

This instruction needs to fail when skipping
a negative amount.  e.g.

  << _:-8, … >> = Foo

should fail the match, not cause a runtime exception.
trifork/erjang,csae1152/erjang,trifork/erjang,trifork/erjang,csae1152/erjang,trifork/erjang,csae1152/erjang,trifork/erjang,trifork/erjang,csae1152/erjang,csae1152/erjang,trifork/erjang