- it is in Go style to panic on memory allocation failure.
- _makechan can already panic, instead of returning NULL, on e.g. ch._mu
  initialization failure.
- we were already not checking in several places for NULL after
  _makechan() call (e.g. in golang/runtime/libgolang_test_c.c).

This switches _makechan to panic on memory allocation failure and
settles on style of future C-level API calls to panic instead of
returning NULL on failure.