When a SP handler is activated, memory is allocated to hold the
MESSAGE_TEXT for the condition that caused the activation.

The problem was that this memory was allocated on the MEM_ROOT belonging
to the stored program. Since this MEM_ROOT is not freed until the
stored program ends, a stored program that causes lots of handler
activations can start using lots of memory. In 5.1 and earlier the
problem did not exist as no MESSAGE_TEXT was allocated if a condition
was raised with a handler present. However, this behavior lead to
a number of other issues such as Bug#23032.

This patch fixes the problem by allocating enough memory for the
necessary MESSAGE_TEXTs in the SP MEM_ROOT when the SP starts and
then re-using this memory each time a handler is activated.
      
This is the 5.5 version of the patch.