Fix hash buffer size in pkey programs

backport from 102a620
diff --git a/programs/pkey/pk_sign.c b/programs/pkey/pk_sign.c
index 7ccc70b..1423494 100644
--- a/programs/pkey/pk_sign.c
+++ b/programs/pkey/pk_sign.c
@@ -74,7 +74,7 @@
     pk_context pk;
     entropy_context entropy;
     ctr_drbg_context ctr_drbg;
-    unsigned char hash[20];
+    unsigned char hash[32];
     unsigned char buf[POLARSSL_MPI_MAX_SIZE];
     char filename[512];
     const char *pers = "pk_sign";