build: Enable -Wnull-dereference warning

This enables `NULL` pointer dereference checking in the compiler. This
isn’t as good as static analysis, but it should hopefully catch some
simple errors without too high a false positive rate.

If the false positive rate is too high to be useful, we can always
disable it again.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #1767

meson.build

@@ -549,6 +549,7 @@ if cc.get_id() == 'gcc' or cc.get_id() == 'clang'
     '-Wmisleading-indentation',
     '-Wmissing-field-initializers',
     '-Wnonnull',
+    '-Wnull-dereference',
     '-Wunused',
     # Due to maintained deprecated code, we do not want to see unused parameters
     '-Wno-unused-parameter',