adb: Use a default _SC_GETPW_R_SIZE_MAX size

sysconf(_SC_GETPW_R_SIZE_MAX) may return −1 if there is no hard limit
on the the buffer size. Some libc implementations such as musl don't
define this limit and will return -1.

Use a default buffer size to handle this case.

Change-Id: I997b13a2c2dca00574e049a259135e61c8ed8e03
Signed-off-by: Robert Yang <decatf@gmail.com>