Always NUL-terminate wxPrintfConvSpec::m_szFlags.

Always NUL-terminate wxPrintfConvSpec::m_szFlags.

The array was initialized and terminating NUL was only added in some
cases. In combination with strchr() calls, this would result it
incorrect calculations or even crashes.

Fixed by initializing the array to zeros. This is less error-prone than
fixing the few places where explicitly adding the terminating NUL was
missing.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64708 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775