message/native_types: Constexpr fix in c++17 mode

If we don't initialize the array at declaration time, we can't consider
the function constexpr as the values could be non-deterministic.

Change-Id: I3ba476cf58b86c44ad2e2216b74da09229e3d0c4
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/message/native_types.cpp b/src/message/native_types.cpp
index 0a81a14..f0a7747 100644
--- a/src/message/native_types.cpp
+++ b/src/message/native_types.cpp
@@ -15,7 +15,7 @@
                                    '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};
 
 constexpr std::array<char, 256> unhex = [] {
-    std::array<char, 256> ret;
+    std::array<char, 256> ret{};
     for (size_t i = 0; i < ret.size(); ++i)
     {
         ret[i] = -1;