ANDROID: proc: fix undefined behavior in proc_uid_base_readdir

When uid_base_stuff has no entries, proc_uid_base_readdir tries to
compute an address before the start of the array. Revise this check to
use uid_base_stuff + nents instead, which makes the code valid
regardless of array size.

Bug: 80158484
Test: No more compiler warning with CONFIG_CPU_FREQ_TIMES=n
Change-Id: I6e55b27c3ba8210cee194f6d27bbd62c0b263796
Signed-off-by: Connor O'Brien <connoro@google.com>