Gracefully handle A_limbs > N_limbs and test it

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/bignum_core.h b/library/bignum_core.h
index cd78e72..f044b33 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -832,7 +832,7 @@
  * When I != NULL (computing the modular inverse), G or I may alias A
  * but none of them may alias N (the modulus).
  *
- * If any precondition is not met, output values are unspecified.
+ * If any of the above preconditions is not met, output values are unspecified.
  *
  * \param[out]    G       The GCD of \p A and \p N.
  *                        Must have the same number of limbs as \p N.