Correct bug with returning 0 for non-empty input from wxConvAuto::ToWChar().

Since the changes of r63064 we could return 0 when asked to convert a
non-empty buffer containing only a BOM. This confused the logic in
wxTextInputStream::NextChar() and was generally unexpected so now return
wxCONV_FAILED in this case.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63245 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/convauto.h

@@ -107,11 +107,11 @@ private:
     void InitFromBOM(BOMType bomType);
 
     // create the correct conversion object for the BOM present in the
-    // beginning of the buffer; adjust the buffer to skip the BOM if found
+    // beginning of the buffer
     //
     // return false if the buffer is too short to allow us to determine if we
     // have BOM or not
-    bool InitFromInput(const char **src, size_t *len);
+    bool InitFromInput(const char *src, size_t len);
 
     // adjust src and len to skip over the BOM (identified by m_bomType) at the
     // start of the buffer