When CONFIG_PASSWORD_DEFAULT is unset, the default_passwd buffer is set
to the empty string. The read_default_passwd() function wants to read at
least two characters from that buffer, causing GCC to generate an array
bounds warning:

barebox/common/password.c: In function 'login':
barebox/common/password.c:173:5: warning: array subscript [1, 2147483647] is outside array bounds of 'const char[1]' [-Warray-bounds]
In file included from barebox/common/password.c:30:
include/generated/passwd.h:1:19: note: while referencing 'default_passwd'

Add an ARRAY_SIZE check to default_passwd so that the loop is optimized
away and the warning is no longer generated. Since the
read_default_passwd() function is only called when default_passwd is not
the empty string, this is not a functional change.

Signed-off-by: David Dgien <dgienda125@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>