slirp/misc: Use the GLib memory allocation APIs

Here we don't check the return value of malloc() which may fail.
Use the g_new() instead, which will abort the program when
there is not enough memory.

Also, use g_strdup instead of strdup and remove the unnecessary
strdup function.

Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Benoît Canet <benoit.canet@nodalink.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
