Fix hash buffer size in pkey programs

backport from 102a620
diff --git a/programs/pkey/dh_client.c b/programs/pkey/dh_client.c
index a5a137a..ea63ce7 100644
--- a/programs/pkey/dh_client.c
+++ b/programs/pkey/dh_client.c
@@ -75,7 +75,7 @@
 
     unsigned char *p, *end;
     unsigned char buf[2048];
-    unsigned char hash[20];
+    unsigned char hash[32];
     const char *pers = "dh_client";
 
     entropy_context entropy;