Fix audioMasterSpecific related segfaults

Not initializing our buffer is fine if something is going to write to
it, but in this case the default way to handle
`audioMasterSpecific` (and any other opcode without specific behaviour)
would be to treat it as a writable string. Since the host wasn't
actually writing to it we would just send some old data from the stack
back which obviously would cause some plugins to crash.

src/common/events.h

@@ -196,7 +196,10 @@ void passthrough_event(boost::asio::local::stream_protocol::socket& socket,
                          event.payload, event.option);
     }
 
-    std::array<char, max_string_length> string_buffer;
+    // This buffer is used to write strings and small objects to. We'll
+    // initialize it with a single null to prevent it from being read as some
+    // arbitrary C-style string.
+    std::array<char, max_string_length> string_buffer{0};
     void* data = std::visit(
         overload{
             [&](const std::nullptr_t&) -> void* { return nullptr; },