Clang 15 warning in bignum.c in 2.28

[gilles-peskine-arm]

In mbedtls-2.28 as of 50bc975 (and since way before 2.28.0), on x86_64 with the default configuration:

$ make CC=clang-15 CFLAGS=-Werror lib
  CC    aes.c
  CC    aesni.c
  CC    arc4.c
  CC    aria.c
  CC    asn1parse.c
  CC    asn1write.c
  CC    base64.c
  CC    bignum.c
bignum.c:1429:29: error: variable 't' set but not used [-Werror,-Wunused-but-set-variable]
    mbedtls_mpi_uint c = 0, t = 0;
                            ^
1 error generated.
make[1]: *** [Makefile:292: bignum.o] Error 1
make: *** [Makefile:17: lib] Error 2

Earlier versions of Clang don't emit a warning.

This doesn't affect development, where this part of the code has been rewritten and the variable t no longer exists.

[gilles-peskine-arm]

Fixed by #7167