…overy.

Using numeric index doesn't work correctly when deferred function have
deferrals inside them along with a blocking call. The test case shows
the exact conditions under which the issue happens and may seem
esoteric, but I've boiled it down from the real code in `net/http`
package.

Fixes gopherjs#1083.

Runtime throws null when it need to unwind the stack from a point where
`panic` was called to where panic was handled. However, if the panic was
thrown in a deferred function and recovered in the subsequent deferred
function of the same call stack frame, this throw went unhandled
crashing the program.

This change adds a point to handle such throws giving a chance to either
stop unwinding or continue invoking deferred functions if another panic
happened.

Fixes gopherjs#780.