Update mpi_gcd_invmod_odd() related comments/documentation

Signed-off-by: Felix Conway <felix.conway@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index 2f2ce6a..e141cda 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1764,7 +1764,7 @@
     }
 
     /* Check aliasing requirements */
-    if (A == N || G == I || (I != NULL && (I == N || G == N))) {
+    if (A == N || (I != NULL && (I == N || G == N))) {
         return MBEDTLS_ERR_MPI_BAD_INPUT_DATA;
     }