Remove some maybe-uninitialized warnings (#5516)

* Remove some maybe-uninitialized warnings

In the Eigen matrix type_caster, resize the matrix instead of assigning with a new one when the matrix size needs to be adjusted.

This can remove lots of compiling warnings about "maybe-uninitialized".

* Revert "Remove some maybe-uninitialized warnings"

This reverts commit 7d5a9b41aa.

* Suppress `-Wmaybe-uninitialized` warning

Reproducer: https://github.com/pybind/pybind11/pull/5516#issuecomment-2645846295

---------

Co-authored-by: Ralf W. Grosse-Kunstleve <rgrossekunst@nvidia.com>

include/pybind11/eigen/matrix.h

@@ -316,8 +316,11 @@ struct type_caster<Type, enable_if_t<is_eigen_dense_plain<Type>::value>> {
             return false;
         }
 
+        PYBIND11_WARNING_PUSH
+        PYBIND11_WARNING_DISABLE_GCC("-Wmaybe-uninitialized") // See PR #5516
         // Allocate the new type, then build a numpy reference into it
         value = Type(fits.rows, fits.cols);
+        PYBIND11_WARNING_POP
         auto ref = reinterpret_steal<array>(eigen_ref_array<props>(value));
         if (dims == 1) {
             ref = ref.squeeze();