Tighten up a lot of casts from unsigned to int which are read by one

of the GET_32BIT macros and then used as length fields. Missing bounds
checks against zero have been added, and also I've introduced a helper
function toint() which casts from unsigned to int in such a way as to
avoid C undefined behaviour, since I'm not sure I trust compilers any
more to do the obviously sensible thing.

[originally from svn r9918]

sshdss.c

@@ -42,7 +42,7 @@ static void getstring(char **data, int *datalen, char **p, int *length)
     *p = NULL;
     if (*datalen < 4)
 	return;
-    *length = GET_32BIT(*data);
+    *length = toint(GET_32BIT(*data));
     if (*length < 0)
         return;
     *datalen -= 4;