staging: rtl8723bs: check for i out of range before accessing szLine[i]

Currently szLine[i] is being accessed before the index i is being
ranged checked.  Fix this by checking the range first.  Also, evaluate
the length of the string szLine just once rather than multiple times and
move the loop variable i to an inner scope and make it an int.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>