py, unix: Allow to compile with -Wsign-compare.

See issue #699.
diff --git a/py/vstr.c b/py/vstr.c
index e87aef9..1f9f136 100644
--- a/py/vstr.c
+++ b/py/vstr.c
@@ -350,7 +350,7 @@
         va_end(ap2);
 
         // if that worked, return
-        if (n > -1 && n < size) {
+        if (n > -1 && (size_t)n < size) {
             vstr->len += n;
             return;
         }