Initialise return values to an error

Initialising the return values to and error is best practice and makes
the library more robust.
diff --git a/library/pk.c b/library/pk.c
index e93ccfd..fc16672 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -30,6 +30,7 @@
 #include "mbedtls/pk_internal.h"
 
 #include "mbedtls/platform_util.h"
+#include "mbedtls/error.h"
 
 #if defined(MBEDTLS_RSA_C)
 #include "mbedtls/rsa.h"
@@ -297,7 +298,7 @@
         mbedtls_ecp_restart_is_enabled() &&
         ctx->pk_info->verify_rs_func != NULL )
     {
-        int ret;
+        int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
         if( ( ret = pk_restart_setup( rs_ctx, ctx->pk_info ) ) != 0 )
             return( ret );
@@ -354,7 +355,7 @@
     if( type == MBEDTLS_PK_RSASSA_PSS )
     {
 #if defined(MBEDTLS_RSA_C) && defined(MBEDTLS_PKCS1_V21)
-        int ret;
+        int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
         const mbedtls_pk_rsassa_pss_options *pss_opts;
 
 #if SIZE_MAX > UINT_MAX
@@ -420,7 +421,7 @@
         mbedtls_ecp_restart_is_enabled() &&
         ctx->pk_info->sign_rs_func != NULL )
     {
-        int ret;
+        int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
         if( ( ret = pk_restart_setup( rs_ctx, ctx->pk_info ) ) != 0 )
             return( ret );
@@ -604,7 +605,7 @@
     psa_ecc_curve_t curve_id;
     psa_key_attributes_t attributes = PSA_KEY_ATTRIBUTES_INIT;
     psa_key_type_t key_type;
-    int ret;
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
 
     /* export the private key material in the format PSA wants */
     if( mbedtls_pk_get_type( pk ) != MBEDTLS_PK_ECKEY )