Fix: wrong casting of return value

The function uri_parse_str_urls return a ssize_t and the return was
casted to a size_t. This caused the following error checking to fail as
-1 was interpreted as 18446744073709551615.

Fixes #803

Signed-off-by: Jonathan Rajotte Julien <jonathan.r.julien@gmail.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>