PR#331: for Visual Studio, use a snprintf/vsnprintf wrapper that ensures the string is terminated.

printbuf.c

@@ -27,6 +27,7 @@
 
 #include "debug.h"
 #include "printbuf.h"
+#include "snprintf_compat.h"
 #include "vasprintf_compat.h"
 
 static int printbuf_extend(struct printbuf *p, int min_size);