mpi_copy: make the 0 case slightly more robust
If Y was constructed through functions in this module, then Y->n == 0
iff Y->p == NULL. However we do not prevent filling mpi structures
manually, and zero may be represented with n=0 and p a valid pointer.
Most of the code can cope with such a representation, but for the
source of mbedtls_mpi_copy, this would cause an integer underflow.
Changing the test for zero from Y->p==NULL to Y->n==0 causes this case
to work at no extra cost.
diff --git a/library/bignum.c b/library/bignum.c
index 9692127..fd9d5b8 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -190,7 +190,7 @@
if( X == Y )
return( 0 );
- if( Y->p == NULL )
+ if( Y->n == 0 )
{
mbedtls_mpi_free( X );
return( 0 );