When all table blocks were removed from the query cache the client session
hung in a tight loop waiting on an impossible condition while consuming a lot
of CPU.

This patch also corrects an error which caused valid tables to sometimes be
removed from the query cache.


mysql-test/r/query_cache.result:
  Added test case to make sure server doesn't hang in a tight loop if last
  table block is removed from the cache.
mysql-test/t/query_cache.test:
  Added test case to make sure server doesn't hang in a tight loop if last
  table block is removed from the cache.
sql/sql_cache.cc:
  - Refactored loop over table blocks. The invalidate_table() function effects
    the elements over which we iterate. The previous stop condition was broken
    due to a compiler optimization error probably caused by the goto-statement
    pointing out of the loop. The effect being that tables_blocks was never
    checked for null values and thus the loop never terminated.
  - The new implementation uses two while loops instead of a goto-statement.
    The tables_blocks is a circular list which becomes null if the last table
    block is removed from the list.