Never spawn more processes than we have pages

This is particularly important for short PDFs on systems with high CPU
count.
Fixes
facebookresearch/nougat#96 (comment)

src/pypdfium2/_helpers/document.py

@@ -625,11 +625,11 @@ def render(
                 raise ValueError("Duplicate page indices are prohibited.")
 
         converter_params = list( inspect.signature(converter).parameters )[1:]
-
         pool_kwargs = dict(
             initializer = _parallel_renderer_init,
             initargs = (self._input, self._password, bool(self.formenv), mk_formconfig, renderer, converter, converter_params, pass_info, kwargs),
         )
+        n_processes = min(n_processes, n_pages)
         with mp.Pool(n_processes, **pool_kwargs) as pool:
             yield from pool.imap(_parallel_renderer_job, page_indices)