Add wxSafeMessageOutput() helper and use it in wxCmdLineParser

No real changes, just remove checking that wxMessageOutput::Get()
returns a non-null pointer as this is something that should really never
happen, but still use wxSafeMessageOutput() checking for this instead,
as this, at least, doesn't look so ugly in the code using it.

include/wx/msgout.h

@@ -49,6 +49,14 @@ private:
     static wxMessageOutput* ms_msgOut;
 };
 
+// This is equivalent to wxMessageOutput::Get()->Output() but doesn't crash if
+// there is no message output object (which shouldn't normally happen).
+inline void wxSafeMessageOutput(const wxString& str)
+{
+    if ( wxMessageOutput* msgOut = wxMessageOutput::Get() )
+        msgOut->Output(str);
+}
+
 // ----------------------------------------------------------------------------
 // helper mix-in for output targets that can use difference encodings
 // ----------------------------------------------------------------------------

interface/wx/msgout.h

@@ -183,3 +183,24 @@ public:
     /// Default constructor.
     wxMessageOutputMessageBox();
 };
+
+/**
+    Check that the message output exists before using it.
+
+    This function is equivalent to
+
+    @code
+    wxMessageOutput::Get()->Output(str);
+    @endcode
+
+    but doesn't do anything if wxMessageOutput::Get() returns @NULL, instead of
+    crashing.
+
+    Note that typically wxMessageOutput::Get() can only ever return @NULL if
+    wxAppTraits::CreateMessageOutput() is overridden to return @NULL, so unless
+    your application does this, using this function is not necessary as the
+    snippet above can be used directly.
+
+    @since 3.3.0
+ */
+void wxSafeMessageOutput(const wxString& str);

src/common/cmdline.cpp

@@ -1236,19 +1236,10 @@ int wxCmdLineParser::Parse(bool showUsage)
     // and also the usage message if it had been requested
     if ( !ok && (!errorMsg.empty() || (helpRequested && showUsage)) )
     {
-        wxMessageOutput* msgOut = wxMessageOutput::Get();
-        if ( msgOut )
-        {
-            wxString usage;
-            if ( showUsage )
-                usage = GetUsageString();
+        if ( showUsage )
+            Usage();
 
-            msgOut->Printf( wxT("%s%s"), usage.c_str(), errorMsg.c_str() );
-        }
-        else
-        {
-            wxFAIL_MSG( wxT("no wxMessageOutput object?") );
-        }
+        wxSafeMessageOutput(errorMsg);
     }
 
     return ok ? 0 : helpRequested ? -1 : 1;
@@ -1260,10 +1251,7 @@ int wxCmdLineParser::Parse(bool showUsage)
 
 void wxCmdLineParser::Usage() const
 {
-    wxMessageOutput* msgOut = wxMessageOutput::Get();
-    wxCHECK_RET( msgOut, wxT("no wxMessageOutput?") );
-
-    msgOut->Output(GetUsageString());
+    wxSafeMessageOutput(GetUsageString(wrapColumn));
 }
 
 wxString wxCmdLineParser::GetUsageString() const