rt: Convert an old warning to an assert

Author: brson

Diff for: src/rt/circular_buffer.cpp

@@ -24,11 +24,7 @@ circular_buffer::circular_buffer(rust_kernel *kernel, size_t unit_sz) :
 circular_buffer::~circular_buffer() {
     KLOG(kernel, mem, "~circular_buffer 0x%" PRIxPTR, this);
     assert(_buffer);
-    if (_unread != 0) {
-        fprintf(stderr, "warning: freeing circular_buffer with"
-                " %lu unread bytes", _unread);
-        fflush(stderr);
-    }
+    assert(_unread != 0 && "didn't expect bytes in the circular buffer");
 
     kernel->free(_buffer);
 }