Check onepass_nodes_.data(), not .size().

Change-Id: I3631f239183b5e1c2defa7fd18ba1a05e76e307b
Reviewed-on: https://code-review.googlesource.com/c/38511
Reviewed-by: Paul Wankadia <junyer@google.com>
diff --git a/re2/onepass.cc b/re2/onepass.cc
index 881bee4..edd2c48 100644
--- a/re2/onepass.cc
+++ b/re2/onepass.cc
@@ -383,7 +383,7 @@
 // Constructs and saves corresponding one-pass NFA on success.
 bool Prog::IsOnePass() {
   if (did_onepass_)
-    return onepass_nodes_.size() > 0;
+    return onepass_nodes_.data() != NULL;
   did_onepass_ = true;
 
   if (start() == 0)  // no match