[TSAN] Rework assertions to always access refcount atomically

`-fsanitize=address` complained that the `refcount > 0` assertions
were reading without atomics.  We can fix this by reworking them
to read the previous value.

Closes: #582
Approved by: jlebon

src/libostree/ostree-repo-commit.c

@@ -2959,7 +2959,8 @@ ostree_repo_commit_modifier_new (OstreeRepoCommitModifierFlags  flags,
 OstreeRepoCommitModifier *
 ostree_repo_commit_modifier_ref (OstreeRepoCommitModifier *modifier)
 {
-  g_atomic_int_inc (&modifier->refcount);
+  gint refcount = g_atomic_int_add (&modifier->refcount, 1);
+  g_assert (refcount > 0);
   return modifier;
 }