#19 Use of assert() requires assert.h/cassert

[Anonymous]

According to the C(++) standard, the assert
macro/function is defined in assert.h. On some systems
(for example, NetBSD) this means compilation fails on
the files mutella/gnushare.cpp and
mutella/gnudownload.cpp because they don't include this
header. (other files which use assert seem to include
the file already)

Easily fixed by adding a simple #include <assert.h> or
#include <cassert> statement to them.