Fix a ref count mistake and add a long-overdue tf_util_print_backtrace() that helped me find it.

src/util.js.h

@@ -126,6 +126,11 @@ const char* tf_util_backtrace_to_string(void* const* buffer, int count);
 */
 const char* tf_util_backtrace_string();
 
+/**
+** Print a stack backtrace of the calling thread.
+*/
+void tf_util_print_backtrace();
+
 /**
 ** Convert a function pointer to its name, if possible.
 ** @return The function name or null.