Address -Wuninitialized-const-pointer with Clang 21

[eramongodb]

Clang 21 was released on 2025-08-26. It seems to have made improvements to its -Wuninitialized diagnostics, which now flag this piece of code:

error: variable 'val' is uninitialized when passed as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]
   43 |    BSON_ASSERT(BCON_EXTRACT(bcon, "foo", BCONE_DECIMAL128(val)));
      |                                                           ^~~

where the BCONE_DECIMAL128(val) expands into:

bcon_extract((bcon),
             "foo",
             bson_bcone_magic(),
             BCON_TYPE_DECIMAL128,
             bcon_ensure_const_decimal128_ptr(&((val))),
//                                            ^~~~~~~~ 
             (void *)NULL)

and bcon_ensure_const_decimal128_ptr has parameter type const bson_decimal128_t *:

The diagnostic reasonably assumes the T in a T const* parameter will be read-from. This is not the case here: bcon_ensure_* is just performing an existence check. However, this is nevertheless triggering our unconditional -Werror=uninitialized flag:

mongo-c-driver/build/cmake/MongoC-Warnings.cmake

Lines 85 to 86 in d04aa62

	# Definite use of uninitialized value
	gnu-like:-Werror=uninitialized msvc:/we4700

[kevinAlbs]

Thank you. LGTM