Use frame class from block, not from context. Fixes #1318.

There's no frame from which to get the class, so we can't get it
from ThreadContext here. Get it from the proc.

Note that there's a more difficult issue here in that there's also
no frame/backtrace/etc on the ThreadContext for invoking a
TracePoint interested in these thread events. Fixing that might
mean making a frame be available at this point, making this fix
unnecessary...but either way the class reference used in this fix
will still be correct.
Use uppercased name to get RubyEvent from TracePoint event.