Fix hash buffer size in pkey programs

backport from 102a620
diff --git a/programs/pkey/rsa_verify_pss.c b/programs/pkey/rsa_verify_pss.c
index 217b713..3e73215 100644
--- a/programs/pkey/rsa_verify_pss.c
+++ b/programs/pkey/rsa_verify_pss.c
@@ -69,7 +69,7 @@
     int ret = 1;
     size_t i;
     pk_context pk;
-    unsigned char hash[20];
+    unsigned char hash[32];
     unsigned char buf[POLARSSL_MPI_MAX_SIZE];
     char filename[512];