Bug 33983 (Stored Procedures: wrong end <label> syntax is accepted)

The server used to crash when REPEAT or another control instruction
was used in conjunction with labels and a LEAVE instruction.

The crash was caused by a missing "pop" of handlers or cursors in the
code representing the stored program. When executing the code in a loop,
this missing "pop" would result in a stack overflow, corrupting memory.

Code generation has been fixed to produce the missing h_pop/c_pop
instructions.

Also, the logic checking that labels at the beginning and the end of a
statement are matched was incorrect, causing Bug 33983.
End labels, when used, must match the label used at the beginning of a block.