max_length parameter for BLOB-returning functions must be big enough
for any possible content. Otherwise the field created for a table
will be too small.


mysql-test/r/gis.result:
  Bug #31158  Spatial, Union, LONGBLOB vs BLOB bug (crops data)
  
  test result
mysql-test/t/gis.test:
  Bug #31158  Spatial, Union, LONGBLOB vs BLOB bug (crops data)
  
  test case
sql/field.cc:
  Bug #31158  Spatial, Union, LONGBLOB vs BLOB bug (crops data)
  
  max_field_size used instead of numeric value
sql/field.h:
  Bug #31158  Spatial, Union, LONGBLOB vs BLOB bug (crops data)
  
  max_field_size constant defined
sql/item_geofunc.cc:
  Bug #31158  Spatial, Union, LONGBLOB vs BLOB bug (crops data)
  
  max_length parameter fixed