str: str_free(NULL) should assert
str: str_free(NULL) should assert

str_free is essentially an internal function.  Therefore, it doesn't have to
be nice to use, and instead it should have extra sanity checking instead.

Signed-off-by: Josef 'Jeff' Sipek <a620f141433c70aa9e43778305a6c68cbfadfb25@josefsipek.net>

jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc,jeffpc/libjeffpc