fastboot: Don't leak file in error case

This is probably not very significant in this standalone tool,
but makes it easier for us to find leaks in our other system
code via static analysis.

(cherry-pick of 407a2195391685627e6be947491041ae3c8cbe61.)

Change-Id: I4e14cadc1e53bac0848e0e0c7f531f920e43cb0a
1 file changed