`-Wnonnull` warnings when configured with `--enable-fields={11,23}`

[hebasto]

A build system is Ubuntu 22.04:

$ g++ --version
g++ (Ubuntu 11.2.0-19ubuntu1) 11.2.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

An excerpt from the build log:

$ ./configure --enable-fields=11 && make clean && make
...
  enable clmul fields     = yes
...
src/fields/clmul_common_impl.h:113:71: warning: ‘this’ pointer is null [-Wnonnull]
  113 |     static inline constexpr I Sqr8(I a) { return SQR8->template Map<O>(a); }
      |                                                  ~~~~~~~~~~~~~~~~~~~~~^~~
...
src/fields/clmul_common_impl.h:114:73: warning: ‘this’ pointer is null [-Wnonnull]
  114 |     static inline constexpr I Sqr16(I a) { return SQR16->template Map<O>(a); }
      |                                                   ~~~~~~~~~~~~~~~~~~~~~~^~~
...