Add nullptr guards in get_internals() for better crash diagnostics

Add explicit null checks after get_pp() and create_pp_content_once()
in get_internals(), calling pybind11_fail() with descriptive messages.
These guards convert potential null-pointer dereferences (which produce
unhelpful access-violation crashes, especially on Windows) into clear
runtime_error messages that can be caught and reported as ImportError
by the try-catch added in the previous commit.

Made-with: Cursor

Author: rwgk

include/pybind11/detail/internals.h

@@ -862,14 +862,21 @@ inline internals_pp_manager<internals> &get_internals_pp_manager() {
 /// Return a reference to the current `internals` data
 PYBIND11_NOINLINE internals &get_internals() {
     auto &ppmgr = get_internals_pp_manager();
-    auto &internals_ptr = *ppmgr.get_pp();
+    auto *pp = ppmgr.get_pp();
+    if (!pp) {
+        pybind11_fail("get_internals: get_pp() returned nullptr");
+    }
+    auto &internals_ptr = *pp;
     if (!internals_ptr) {
         // Slow path, something needs fetched from the state dict or created
         gil_scoped_acquire_simple gil;
         error_scope err_scope;
 
         ppmgr.create_pp_content_once(&internals_ptr);
 
+        if (!internals_ptr) {
+            pybind11_fail("get_internals: create_pp_content_once() produced nullptr");
+        }
         if (!internals_ptr->instance_base) {
             // This calls get_internals, so cannot be called from within the internals constructor
             // called above because internals_ptr must be set before get_internals is called again