Fix compile warning in tor_inet_pton() (on 64bit)

This slipped through into 0.2.3.8-alpha unfortunately.

changes/bug4554

@@ -0,0 +1,4 @@
+  o Minor bugfixes:
+    - Fix a compile warning in tor_inet_pton(). Bugfix on 0.2.3.8-alpha;
+      fixes bug 4554.
+

src/common/compat.c

@@ -1733,7 +1733,7 @@ tor_inet_pton(int af, const char *src, void *dst)
         return 0;
       if (TOR_ISXDIGIT(*src)) {
         char *next;
-        int len;
+        ssize_t len;
         long r = strtol(src, &next, 16);
         tor_assert(next != NULL);
         tor_assert(next != src);