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 moved the memory allocation code block (lines 69-73) just before
the line 102. This is the first place the pointer reason_codes is used, while
the following mosquitto__free operators free the allocated memory correctly.

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