Do not actually Kill in debug_break() in non-debug binaries

Revelead by the A23 crash when hosting a game in the lobby. We have a
debug_break() call that calls "kill" with SIGTRAP, which generally
doesn't get handled correctly in release versions, particularly on OSX.

Differential Revision: https://code.wildfiregames.com/D1499
This was SVN commit r21998.

source/lib/sysdep/os/unix/udbg.cpp

@@ -47,7 +47,9 @@ Status debug_CaptureContext(void* UNUSED(context))
 
 void debug_break()
 {
+#ifndef NDEBUG
 	kill(getpid(), SIGTRAP);
+#endif
 }
 
 #define DEBUGGER_WAIT 3