lineedit: stop using permanent int_buf[] (16k!): allocate it

lineedit: stop using permanent int_buf[] (16k!): allocate it

Now it is allocated temporarily only for the duretion of prefix generation,
and also we only allocate the needed size, not maximally possible.

function                                             old     new   delta
build_match_prefix                                   579     590     +11
remove_chunk                                          43      28     -15
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 11/-15)             Total: -4 bytes

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>