[C++] Make sure all members are initilized

Compiler dead store elimination optimization pass should remove any
unncessary  stores  when the codecs are used correctly. With incorrect use
it will give predictable behavior.

Author: rigtorp

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppGenerator.java

@@ -197,14 +197,14 @@ private static void generateGroupClassHeader(
             indent + "class %1$s\n" +
             indent + "{\n" +
             indent + "private:\n" +
-            indent + "    char *m_buffer;\n" +
-            indent + "    std::uint64_t m_bufferLength;\n" +
-            indent + "    std::uint64_t *m_positionPtr;\n" +
-            indent + "    std::uint64_t m_blockLength;\n" +
-            indent + "    std::uint64_t m_count;\n" +
-            indent + "    std::uint64_t m_index;\n" +
-            indent + "    std::uint64_t m_offset;\n" +
-            indent + "    std::uint64_t m_actingVersion;\n\n" +
+            indent + "    char *m_buffer = nullptr;\n" +
+            indent + "    std::uint64_t m_bufferLength = 0;\n" +
+            indent + "    std::uint64_t *m_positionPtr = nullptr;\n" +
+            indent + "    std::uint64_t m_blockLength = 0;\n" +
+            indent + "    std::uint64_t m_count = 0;\n" +
+            indent + "    std::uint64_t m_index = 0;\n" +
+            indent + "    std::uint64_t m_offset = 0;\n" +
+            indent + "    std::uint64_t m_actingVersion = 0;\n\n" +
 
             indent + "    SBE_NODISCARD std::uint64_t *sbePositionPtr() SBE_NOEXCEPT\n" +
             indent + "    {\n" +
@@ -1619,9 +1619,9 @@ private CharSequence generateFixedFlyweightCode(final String className, final in
         return String.format(
             "private:\n" +
             "    char *m_buffer = nullptr;\n" +
-            "    std::uint64_t m_bufferLength;\n" +
+            "    std::uint64_t m_bufferLength = 0;\n" +
             "    std::uint64_t m_offset = 0;\n" +
-            "    std::uint64_t m_actingVersion;\n\n" +
+            "    std::uint64_t m_actingVersion = 0;\n\n" +
 
             "public:\n" +
             "    enum MetaAttribute\n" +
@@ -1772,8 +1772,8 @@ private CharSequence generateMessageFlyweightCode(final String className, final
             "    char *m_buffer = nullptr;\n" +
             "    std::uint64_t m_bufferLength = 0;\n" +
             "    std::uint64_t m_offset = 0;\n" +
-            "    std::uint64_t m_position;\n" +
-            "    std::uint64_t m_actingVersion;\n\n" +
+            "    std::uint64_t m_position = 0;\n" +
+            "    std::uint64_t m_actingVersion = 0;\n\n" +
 
             "    inline std::uint64_t *sbePositionPtr() SBE_NOEXCEPT\n" +
             "    {\n" +