bootable/recovery: cleanup compiler warnings (unused value)

bootable/recovery/applypatch/imgdiff.cpp:322:11: warning: Value stored to 'ret' during its initialization is never read [clang-analyzer-deadcode.DeadStores]
bootable/recovery/applypatch/imgdiff.cpp:447:11: warning: Value stored to 'ret' during its initialization is never read [clang-analyzer-deadcode.DeadStores]
bootable/recovery/applypatch/imgdiff.cpp:553:3: warning: Value stored to 'ret' is never read [clang-analyzer-deadcode.DeadStores]

Bug: 26936282
Test: WITH_TIDY=1 WITH_STATIC_ANALYZER=1 mm
Change-Id: I3f865e3e9b9d19e5ea5e8dfd2fe2c644254ffbb5