MDEV-17133 follow-up patch to fix mf_iocache-t unittest

which did not always correctly simulated io-cache::end_of_file.
The error was caused by implicit cast to unsigned of an intemediate term
in a formula.

unittest/sql/mf_iocache-t.cc

@@ -326,7 +326,7 @@ void mdev17133()
       MY_MIN(sizeof(buf_o),
              info.end_of_file + eof_block_size +
              // plus 25% of block for randomization to the average
-             (eof_block_size/4 - rand() % (eof_block_size/2)));
+             eof_block_size/4 - rand() % (eof_block_size/2));
 
     // read a chunk by blocks of variable size read_iter times
     // the last block completes the current chunk