tools/mm/slabinfo: fix access to null terminator in string boundary

tools/mm/slabinfo: fix access to null terminator in string boundary

The current code incorrectly accesses buffer[strlen(buffer)], which points
to the null terminator ('\0') at the end of the string.  This is
technically out-of-bounds access since valid string content ends at index
strlen(buffer)-1.

Fix by:
1. Declaring strlen() result variable at function scope
2. Adding bounds check (len > 0) to handle empty strings
3. Using buffer[len-1] to correctly access the last character before
   the null terminator

[kaushlendra.kumar@intel.com: remove unnecessary blank line]
Link: https://lkml.kernel.org/r/20250901044955.3902815-1-kaushlendra.kumar@intel.com
Link: https://lkml.kernel.org/r/20250830172022.1927448-1-kaushlendra.kumar@intel.com
Signed-off-by: Kaushlendra Kumar <kaushlendra.kumar@intel.com>
Acked-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>