Fix assertion failure on failed magic eval - eg FETCH {eval'('}

Fix assertion failure on failed magic eval - eg FETCH {eval'('}
S_doeval()'s behaviour varies depending on whether the code
compiles or not; on failure it pops the EVAL context block. This
is bad because later on, S_docatch() assumes that the block is
still there. Make docatch() return a boolean instead, indicating
success. The value it formerly returned (the next op) can be deduced
as PL_eval_start or PL_op->op_next on success/failure.

p4raw-id: //depot/perl@31582