win32: make vlc_vsnprintf more like c99 vsnprintf
win32: make vlc_vsnprintf more like c99 vsnprintf

Fixes following issues using MSVCRT _snprintf:
 - Failure to null terminate all strings
 - Failure to return number of characters that would've been
   printed had the buffer been sufficiently large
 - Failure to accept size = 0 (and str = NULL) to determine
   final output length.

NB, the third issue above is fixed on *some* win32 implementations,
however it is not officially documented as being so.

Signed-off-by: David Flynn <38ae1b48ed96d57c9e166f6cc6ffdf619b4856ab@rd.bbc.co.uk>
Signed-off-by: Jean-Baptiste Kempf <7b85a41a628204b76aba4326273a3ccc74bd009a@videolan.org>

shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,krichter722/vlc,krichter722/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.2,xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,krichter722/vlc,xkfz007/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.1,xkfz007/vlc,shyamalschandra/vlc,xkfz007/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,shyamalschandra/vlc,shyamalschandra/vlc,krichter722/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,vlc-mirror/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc,shyamalschandra/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,xkfz007/vlc,shyamalschandra/vlc,xkfz007/vlc