[perl #123753] Fix assertion failure with map+map

[perl #123753] Fix assertion failure with map+map

ck_grep calls ck_fun, which sets the lower private bits to indicate
the number of operands.  ck_grep usually undoes that, by clobbering
op_private completetly.  If an error has occurred, it doesn’t, so we
may fail an assertion if the lower bits are not expected to be set on
mapstart and grepstart.