bignum: follow customs for ret initialisation

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/bignum.c b/library/bignum.c
index 16b1449..ccccff5 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2014,7 +2014,7 @@
                                              mbedtls_mpi const *A,
                                              mbedtls_mpi const *N)
 {
-    int ret;
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     mbedtls_mpi I, G;
 
     mbedtls_mpi_init(&I);
@@ -2054,7 +2054,7 @@
                                     mbedtls_mpi const *A,
                                     mbedtls_mpi const *N)
 {
-    int ret;
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
     mbedtls_mpi AA;
 
     mbedtls_mpi_init(&AA);