Add assertion for index 'a' in suffix array

Add assertion to ensure 'a' is within bounds before accessing 'y'.

Author: lrvideckis

library/strings/suffix_array/suffix_array_short.hpp

@@ -23,7 +23,7 @@ auto sa_short(const auto& s) {
         int a = i1 + j * k < n ? x[i1 + j * k] : -1;
         int b = i2 + j * k < n ? x[i2 + j * k] : -1;
         if (a != b) {
-          if (a != -1) val += y[a];
+          if (a != -1) assert(a<n-1),val += y[a];
           else val = n - i1;
           return a < b;
         }