added wxVaCopy() and use it to fix wxVsnprintf() crash (see bug 1017651)

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@29604 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/latex/wx/function.tex

@@ -237,6 +237,7 @@ the corresponding topic.
 \helpref{wxUnix2DosFilename}{wxunix2dosfilename}\\
 \helpref{wxUnsetEnv}{wxunsetenv}\\
 \helpref{wxUsleep}{wxusleep}\\
+\helpref{wxVaCopy}{wxvacopy}\\
 \helpref{wxVsnprintf}{wxvsnprintf}\\
 \helpref{wxWakeUpIdle}{wxwakeupidle}\\
 \helpref{wxWriteResource}{wxwriteresource}\\
@@ -2977,6 +2978,19 @@ allows to define unsigned 64 bit compile time constants:
 \helpref{wxLL}{wxll}, \helpref{wxLongLong}{wxlonglong}
 
 
+\membersection{wxVaCopy}\label{wxvacopy}
+
+\func{void}{wxVaCopy}{\param{va\_list }{argptrDst}, \param{va\_list}{argptrSrc}}
+
+This macro is the same as the standard C99 \texttt{va\_copy} for the compilers
+which support it or its replacement for those that don't. It must be used to
+preserve the value of a \texttt{va\_list} object if you need to use it after
+passing it to another function because it can be modified by the latter.
+
+As with \texttt{va\_start}, each call to \texttt{wxVaCopy} must have a matching 
+\texttt{va\_end}.
+
+
 \membersection{::wxWriteResource}\label{wxwriteresource}
 
 \func{bool}{wxWriteResource}{\param{const wxString\& }{section}, \param{const wxString\& }{entry},