Fix STL header writing (really)

The vsnprintf call must be passed the buffer size, not the target string size. Shortening the string to field length happens in the memcpy.

src/subtitle_format_ebu3264.cpp

@@ -531,7 +531,7 @@ namespace
 		char buf[16];
 		va_list ap;
 		va_start(ap, format);
-		vsnprintf(buf, fieldlen, format, ap);
+		vsnprintf(buf, sizeof(buf), format, ap);
 		va_end(ap);
 		memcpy(field, buf, fieldlen);
 	}