If a dict would include duplicated items, the parsing code here would
have leaked memory by overwriting old os_strdup() result with the new
one. Fix this by explicitly freeing the previous entry. This addresses
CID 62852, CID 62851, CID 62850, CID 62849, CID 62847, CID 62846.
Signed-off-by: Jouni Malinen <j@w1.fi>