client.c: fix -Wstringop-overflow warning in cmdline_token()
client.c: fix -Wstringop-overflow warning in cmdline_token()

Calling strncpy() on the variable (and erroneous) calculated token
length in cmdline_token() has caused stringop-overflow warning by
GCC 8. This change fixes the error and replaces strncpy() by
memcpy() to quieten the compiler.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap