Silence a warning from Clang >=15 about an unused local variable

The assembly code uses t only on some architectures. Fixes #7166.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index d915ade..5dca3a9 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1427,6 +1427,7 @@
                  mbedtls_mpi_uint b)
 {
     mbedtls_mpi_uint c = 0, t = 0;
+    (void) t;                   /* Unused in some architectures */
 
 #if defined(MULADDC_HUIT)
     for (; i >= 8; i -= 8) {
@@ -1472,8 +1473,6 @@
     }
 #endif /* MULADDC_HUIT */
 
-    t++;
-
     while (c != 0) {
         *d += c; c = (*d < c); d++;
     }