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]

sshpubk.c

@@ -73,7 +73,7 @@ static int loadrsakey_main(FILE * fp, struct RSAKey *key, int pub_only,
     i += j;
 
     /* Next, the comment field. */
-    j = GET_32BIT(buf + i);
+    j = toint(GET_32BIT(buf + i));
     i += 4;
     if (j < 0 || len - i < j)
 	goto end;