MVE: Make vsnprinf() usage more robust

Use sizeof() instead of hardcoding string length when calling
vsnprintf().

Fixes: 247ea30 ("MVE: Replace vsprintf() with vsnprintf()")

libs/mve/image_io.cc

@@ -631,7 +631,7 @@ void
 tiff_error_handler (char const* /*module*/, char const* fmt, va_list ap)
 {
     char msg[2048];
-    ::vsnprintf(msg, 2048, fmt, ap);
+    ::vsnprintf(msg, sizeof(msg), fmt, ap);
     throw util::Exception(msg);
 }