Fix hypothetical double-own in hidl_vec.

To my knowledge, no one has hit this, but this type (probably unwisely)
allows itself to be empty w/ null buffer or empty w/ non-null buffer
(e.g. via setExternal/resize(0)). So, releaseData should be checking for
null buffer, not size 0.

Bug: 152911913
Test: libhidl_test
Change-Id: I5792b5aac5b3278333a75ff84f09d3d5e5b3a8a2
2 files changed