With GCC 6, the strdup() function is declared with the "nonnull"
attribute, stating that it is not allowed to pass a NULL value as
parameter.
In nedmalloc()'s reimplementation of strdup(), Postel's Law is heeded
and NULL parameters are handled gracefully. GCC 6 complains about that
now because it thinks that NULL cannot be passed to strdup() anyway.
Let's just shut up GCC >= 6 in that case and go on with our lives.
See https://gcc.gnu.org/gcc-6/porting_to.html for details.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>