Fix SSLv3 handling of SHA-384 suites

Fixes memory corruption, introduced in
a5bdfcd (Relax some SHA2 ciphersuite's version requirements)
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index c1e3d37..a903b3e 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -941,6 +941,8 @@
         padlen = 40;
     else if( md_type == POLARSSL_MD_SHA256 )
         padlen = 32;
+    else if( md_type == POLARSSL_MD_SHA384 )
+        padlen = 16;
 
     memcpy( header, ctr, 8 );
     header[ 8] = (unsigned char)  type;