Do not pass nullptrs to memcpy

It is undefined behavior.

Change-Id: I8380f1af90bbaffb39488bb1367427b3906d2526
Signed-off-by: James Kuszmaul <james.kuszmaul@bluerivertech.com>
diff --git a/aos/flatbuffers.h b/aos/flatbuffers.h
index bea1225..ed9c8ac 100644
--- a/aos/flatbuffers.h
+++ b/aos/flatbuffers.h
@@ -188,6 +188,7 @@
   // Builds a Flatbuffer by copying the data from the other flatbuffer.
   FlatbufferVector(const NonSizePrefixedFlatbuffer<T> &other) {
     data_.resize(other.span().size());
+    CHECK(other.span().data());
     memcpy(data_.data(), other.span().data(), data_.size());
   }