USB: gadget: mtp: Fix bug in freeing memory on allocation failure

Current code doesn't handle the memory allocation failure
correctly and ends up leaking memory and trying to free
unallocated one.

CRs-fixed: 540226
Change-Id: I7e3527b7e08ccfe566d85e3009d8a015d8daa707
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>