remote: avoid -Wunused-but-set-variable in gcc with -DNDEBUG

remote: avoid -Wunused-but-set-variable in gcc with -DNDEBUG

In make_remote(), we store the return value of hashmap_put() and check
it using assert(), but don't otherwise use it. If Git is compiled with
NDEBUG, then the assert() becomes a noop, and nobody looks at the
variable at all. This causes some compilers to produce warnings.

Let's switch it instead to a BUG(). This accomplishes the same thing,
but is always compiled in (and we don't have to worry about the cost;
the check is cheap, and this is not a hot code path).

Signed-off-by: Carlo Marcelo Arenas Belón <redacted>
Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>