This may lead to buffer overflow.
The String::qs_append() function will append a string
without checking if there's enough space.
So qs_append() must be called beforehand to ensure 
there's enough space in the buffer for the subsequent 
qs_append() calls.
Fixed Item_case_expr::print() to make sure there's
enough space before appending data by adding a call to 
String::reserve() to make sure qs_append() will have 
enough space.


mysql-test/r/sp-code.result:
  Bug #26303: test case
mysql-test/t/sp-code.test:
  Bug #26303: test case
sql/item.cc:
  Bug #26303: added a call to String::reserve() to
   make sure qs_append will have enough space
sql/item.h:
  Bug #26303: m_case_expr_id made unsigned 
  because it's offset in an array.