CCBC-1674: Do not use non-owning spans to release memory with NETBUF_LIBC_PROXY

Fix use-after-free for build with NETBUF_LIBC_PROXY when snappy
compression is enabled. This is special build when caching allocator
is replaced with just libc malloc. It is used only during
development and testing, and never enabled in production.

Change-Id: If26d33c41819a475e99b40f9e6dddc3c15186170
Reviewed-on: https://review.couchbase.org/c/libcouchbase/+/232551
Reviewed-by: Michael Reiche <michael.reiche@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>

Author: avsej

src/netbuf/netbuf.c

@@ -682,7 +682,9 @@ void netbuf_end_flush2(nb_MGR *mgr, unsigned int nflushed, nb_getsize_fn callbac
 void netbuf_mblock_release(nb_MGR *mgr, nb_SPAN *span)
 {
 #ifdef NETBUF_LIBC_PROXY
-    free(span->parent);
+    if (span->offset == 0) {
+        free(span->parent);
+    }
     (void)mgr;
 #else
     mblock_release_data(&mgr->datapool, span->parent, span->size, span->offset);