build: handle nil Options in largeFilesFlag
build: handle nil Options in largeFilesFlag

The flag package may call the String method with a zero-valued receiver,
such as a nil pointer. For example this happens when passing in the flag
"-help", so this fixes a panic in that case. This was a panic introduced
in 5dd4713 build: factor out build flags.

Change-Id: I27e5b66637fe2f3ad8231baa3e5b355b579e052f

google/zoekt,google/zoekt