BUG/MINOR: http_rules: fix errors paths in http_parse_redirect_rule()

BUG/MINOR: http_rules: fix errors paths in http_parse_redirect_rule()

http_parse_redirect_rule() doesn't perform enough checks around NULL
returning allocating functions.

Moreover, existing error paths don't perform cleanups. This could lead to
memory leaks.

Adding a few checks and a cleanup path to ensure memory errors are
properly handled and that no memory leaks occurs within the function
(already allocated structures are freed on error path).

It should partially fix GH #2130.

This patch depends on ("MINOR: proxy: add http_free_redirect_rule() function")

This could be backported up to 2.4. The patch is also relevant for
2.2 but "MINOR: proxy: add http_free_redirect_rule() function" would
need to be adapted first.

==

Backport notes:

-> For 2.2 only:

Replace:

    (strcmp(args[cur_arg], "drop-query") == 0)

with:

    (!strcmp(args[cur_arg],"drop-query"))

-> For 2.2 and 2.4:

Replace:

    "expects 'code', 'prefix', 'location', 'scheme', 'set-cookie', 'clear-cookie', 'drop-query', 'ignore-empty' or 'append-slash' (was '%s')",

with:

    "expects 'code', 'prefix', 'location', 'scheme', 'set-cookie', 'clear-cookie', 'drop-query' or 'append-slash' (was '%s')",