pppd: Don't try to free(NULL - 1)

pppd: Don't try to free(NULL - 1)

A logic bug in update_script_environment() means that it can call
remove_script_env() even when the variable being removed is not
present in the script_env array.  The result of that is that
remove_script_env() will call free() with argument NULL - 1.
To fix this, we avoid calling remove_script_env() in this case.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>