CHECK TABLE could complain about a fully intact spatial index.
A wrong comparison operator was used for table checking. 
The result was that it checked for non-matching spatial keys. 
This succeeded if at least two different keys were present, 
but failed if only the matching key was present.

I fixed the key comparison.


myisam/mi_check.c:
  Bug#17877 - Corrupted spatial index
  Fixed the comparison operator for checking a spatial index.
  Using MBR_EQUAL | MBR_DATA to compare for equality and
  include the data pointer in the comparison. The latter
  finds the index entry that points to the current record.
  This is necessary for non-unique indexes.
  
  The old operator, SEARCH_SAME, is unknown to the rtree
  search functions and handled like MBR_DISJOINT.
myisam/mi_key.c:
  Bug#17877 - Corrupted spatial index
  Added a missing DBUG_RETURN.
myisam/rt_index.c:
  Bug#17877 - Corrupted spatial index
  Included the data pointer in the copy of the search key.
  This is necessary for searching the index entry that points
  to a specific record if the search_flag contains MBR_DATA.
myisam/rt_mbr.c:
  Bug#17877 - Corrupted spatial index
  Extended the RT_CMP() macro with an assert for an 
  unexpected comparison operator.
mysql-test/r/gis-rtree.result:
  Bug#17877 - Corrupted spatial index
  The test result.
mysql-test/t/gis-rtree.test:
  Bug#17877 - Corrupted spatial index
  The test case.