Do not leave empty, invalid chunks in buffers during buf_pullup

This fixes an assertion failure bug in 15083; bugfix on 0.2.0.10-alpha.

Patch from 'cypherpunks'

changes/bug15083

+  o Major bugfixes (relay, stability):
+    - Fix a bug that could lead to a relay crashing with an assertion
+      failure if a buffer of exactly the wrong layout was passed
+      to buf_pullup() at exactly the wrong time. Fixes bug 15083;
+      bugfix on 0.2.0.10-alpha. Patch from 'cypherpunks'.
+   

src/or/buffers.c

@@ -426,7 +426,7 @@ buf_pullup(buf_t *buf, size_t bytes, int nulterminate)
     size_t n = bytes - dest->datalen;
     src = dest->next;
     tor_assert(src);
-    if (n > src->datalen) {
+    if (n >= src->datalen) {
       memcpy(CHUNK_WRITE_PTR(dest), src->data, src->datalen);
       dest->datalen += src->datalen;
       dest->next = src->next;