causing update of a different column

Post-pushbuild fix.

bitmap_set_bit() is an inline function in DEBUG builds and
a macro in non-DEBUG builds. The latter evaluates its 'bit'
argument twice. So one must not use increment/decrement operators
on this argument.

Moved increment of pointer out of bitmap_set_bit() call.


include/my_bitmap.h:
  Bug#26827 - table->read_set is set incorrectly,
              causing update of a different column
  Added a warning comment.
sql/sql_partition.cc:
  Bug#26827 - table->read_set is set incorrectly,
              causing update of a different column
  Moved increment of pointer out of bitmap_set_bit() call.