kex_agree_hostkey: fix NULL pointer derefence

While setting up the session, ssh tries to determine the type of
encryption method it can use for the session. This requires looking at
the keys offered by the remote host and comparing these with the methods
supported by libssh2 (rsa & dss). To do this there is an iteration over
the array containing the methods supported by libssh2.

If there is no agreement on the type of encryption we come to the 3rd
entry of the hostkeyp array. Here hostkeyp is valid but *hostkep is
NULL. Thus when we dereference that in (*hostkeyp)->name there is a
crash

src/kex.c

@@ -1273,7 +1273,7 @@ static int kex_agree_hostkey(LIBSSH2_SESSION * session,
         return -1;
     }
 
-    while (hostkeyp && (*hostkeyp)->name) {
+    while (hostkeyp && (*hostkeyp) && (*hostkeyp)->name) {
         s = kex_agree_instr(hostkey, hostkey_len,
                             (unsigned char *) (*hostkeyp)->name,
                             strlen((*hostkeyp)->name));