Problem was that it tried to run partitioning function calls when
opening a partitioned table, when it was explicitly disabled.

Solution is to check if the partitioning plugin is ready to use before
using any partitioning specific calls.

mysql-test/r/disabled_partition.require:
  Bug#39893: Crash if select on a partitioned table, when partitioning is disabled
  
  New require file to use when partitioning is disabled (but compiled in)
mysql-test/r/not_partition.result:
  Bug#39893: Crash if select on a partitioned table, when partitioning is disabled
  
  Updated testcase
mysql-test/r/partition_disabled.result:
  Bug#39893: Crash if select on a partitioned table, when partitioning is disabled
  
  New result file
mysql-test/std_data/parts/t1.frm:
  Bug#39893: Crash if select on a partitioned table, when partitioning is disabled
  
  frm file for 'create table t1 (a int) partition by hash (a)'
mysql-test/t/not_partition.test:
  Bug#39893: Crash if select on a partitioned table, when partitioning is disabled
  
  Updated test cases
mysql-test/t/partition_disabled-master.opt:
  Bug#39893: Crash if select on a partitioned table, when partitioning is disabled
  
  New opt file
mysql-test/t/partition_disabled.test:
  Bug#39893: Crash if select on a partitioned table, when partitioning is disabled
  
  New test file (looks like not_partition.test, but with different errors)
sql/sql_yacc.yy:
  Bug#39893: Crash if select on a partitioned table, when partitioning is disabled
  
  Better error message (it is already built with partitioning, but is explicitly
  disabled).
sql/table.cc:
  Bug#39893: Crash if select on a partitioned table, when partitioning is disabled
  
  If the partitioning plugin is not ready, fail to open the table.