Memory leak in handle_unsubscribe.c

Reason: In line 70, the memory allocation for the pointer reasons_codes may
result to a memory leak due to the many returns (e.g as the one in line 78)
occuring in the program's path until reaching the mosquitto__free at line 122.

Fix: I added a mosquitto__free(reason_codes) statement before each return
statement that could result to a memory leak

Signed-off-by: Panagiotis Vasilikos <panagiotis.vasilikos@alexandra.dk>