bsdinstall/distfetch.c: check environment variables before downloading and handle...

bsdinstall/distfetch.c: check environment variables before downloading and handle memory allocation errors

1. Currently, distfetch checks environment variables existence
when it will use them or in a case (in chdir()) it doesn't check
at all. As they are necessary to set before doing anything with
it, check them, if they set or not, before proceeding any further.
This also avoids extra cleaning when that environment variable
isn't set.

2. Handle memory allocation error in malloc(PATH_MAX) and replace
(sizeof const char *) with (sizeof char *). Both are similar and
const doesn't have a size.

3. Indent the error message a bit in chdir().

Signed-off-by: rilysh <nightquick@proton.me>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1071