Occurred with EXTRA_DEBUG on windows.

Problem was insufficient length of a local variable that stored path names.

Solution was to use the correct length.

CMakeLists.txt:
  Bug#38784: Mysql server crash if table is altered with partition changes.
  
  Added support for EXTRA_DEBUG
sql/sql_partition.cc:
  Bug#38784: Mysql server crash if table is altered with partition changes.
  
  Changed from FN_LEN to FN_REFLEN since the variable was use for paths,
  not filenames without path.
win/configure.js:
  Bug#38784: Mysql server crash if table is altered with partition changes.
  
  Added support for EXTRA_DEBUG