Adds a check for "digits only" within the length field of a "netstring".
BUG: http://www.guninski.com/qmail-qmtpd.html

diff -ruN qmail-1.03-7.052/qmail-qmtpd.c qmail-1.03-7.051/qmail-qmtpd.c
--- qmail-1.03-7.052/qmail-qmtpd.c	1998-06-15 06:53:16.000000000 -0400
+++ qmail-1.03-7.051/qmail-qmtpd.c	2008-01-23 09:27:30.000000000 -0500
@@ -45,6 +45,8 @@
   for (;;) {
     substdio_get(&ssin,&ch,1);
     if (ch == ':') return len;
+    /* trap non-numeric input in netstring: */
+    if ((ch < '0') || (ch > '9')) badproto();
     if (len > 200000000) resources();
     len = 10 * len + (ch - '0');
   }
@@ -193,6 +195,8 @@
         substdio_get(&ssin,&ch,1);
         --biglen;
         if (ch == ':') break;
+        /* trap non-numeric input in netstring: */
+        if ((ch < '0') || (ch > '9')) badproto();
         if (len > 200000000) resources();
         len = 10 * len + (ch - '0');
       }