Ensure DDTRACE_G(active_stack) is always NULL when the root span is b…

…eing pushed

I have zero idea _how_ this happened, given that I'm unable to reproduce it, but a core dump indicated that the memory it pointed to was from the previous request.
While this is not a very proper fix (as this should not be possible in the first place), it mitigates the effects.

Signed-off-by: Bob Weinand <bob.weinand@datadoghq.com>

ext/ddtrace.c

@@ -1302,6 +1302,7 @@ static PHP_RINIT_FUNCTION(ddtrace) {
     if (!ddtrace_disable) {
         // With internal functions also being hookable, they must not be hooked before the CG(map_ptr_base) is zeroed
         zai_hook_activate();
+        DDTRACE_G(active_stack) = NULL; // This should not be necessary, but somehow sometimes it may be a leftover from a previous request.
         DDTRACE_G(active_stack) = ddtrace_init_root_span_stack();
 #if PHP_VERSION_ID < 80000
         ddtrace_autoload_rinit();