libgolang: Switch _makechan to panic instead of return NULL on failure

- 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.

golang/libgolang.h

@@ -359,8 +359,6 @@ chan<T> makechan(unsigned size) {
         ? 0          // eg struct{} for which sizeof() gives 1 - *not* 0
         : sizeof(T);
     ch._ch = _makechan(elemsize, size);
-    if (ch._ch == NULL)
-        throw std::bad_alloc();
     return ch;
 }
 

golang/runtime/libgolang.cpp

@@ -417,11 +417,12 @@ _RecvSendWaiting *_dequeWaiter(list_head *queue) {
 // _makechan creates new _chan(elemsize, size).
 //
 // returned channel has refcnt=1.
+// _makechan always returns !NULL and panics on memory allocation failure.
 _chan *_makechan(unsigned elemsize, unsigned size) {
     _chan *ch;
     ch = (_chan *)zalloc(sizeof(_chan) + size*elemsize);
     if (ch == NULL)
-        return NULL;
+        panic("makechan: alloc failed");
     new (&ch->_mu) Mutex();
 
     ch->_refcnt   = 1;

golang/runtime/libgolang_test_c.c

@@ -74,8 +74,6 @@ struct _work_arg{int i; _chan *done;};
 static void _work(void *);
 void _test_go_c(void) {
     _chan *done = _makechan(0,0);
-    if (done == NULL)
-        panic("_makechan -> failed");
     struct _work_arg *_ = malloc(sizeof(*_));
     if (_ == NULL)
         panic("malloc _work_arg -> failed");