afh_recv: Check return value of afh_get_chunk().

afh_recv: Check return value of afh_get_chunk().

Starting with commit b6b571e6c (afh: Dynamic chunks) the function
returns an int value and may fail. Although we store the return value
in a variable, the value of the variable is never read, causing the
clang static analyzer to complain (rightfully).

This patch modifies afh_recv_post_select() to fail if afh_get_chunk()
returns negative.