When analyzing the possible index use cases the server was re-using an internal structure.
This is wrong, as this internal structure gets updated during the analysis.
Fixed by making a copy of the internal structure for every place it needs to be used.
Also stopped the generation of empty SEL_TREE structures that unnecessary 
complicate the analysis.

mysql-test/r/index_merge.result:
  Bug#37943: test case
mysql-test/t/index_merge.test:
  Bug#37943: test case
sql/opt_range.cc:
  Bug#37943: 
   - Make copy constructors for SEL_TREE and sub-structures and use them when OR-ing trees.
   - don't generate empty SEL_TREEs. Return NULL instead.