gdb: Only paginate for filtered output in fputs_maybe_filtered

A have had situation where a unfiltered output (done using
fputs_unfiltered) ended up triggering pagination.  The backtrace for this was:

    ...
    #24 0x000055839377ee4e in check_async_event_handlers () at ../../gdb/async-event.c:335
    #25 0x0000558394b67b57 in gdb_do_one_event () at ../../gdbsupport/event-loop.cc:216
    #26 0x0000558394587454 in gdb_readline_wrapper (prompt=0x7ffd907712d0 "--Type <RET> for more, q to quit, c to continue without paging--") at ../../gdb/top.c:1148
    #27 0x0000558394707270 in prompt_for_continue () at ../../gdb/utils.c:1438
    #28 0x00005583947088b3 in fputs_maybe_filtered (linebuffer=0x60c0000f4000 "   [...quite big message...]", stream=0x60300028e9d0, filter=0) at ../../gdb/utils.c:1752
    #29 0x0000558394708e57 in fputs_unfiltered (linebuffer=0x60c0000f4000 "   [...quite big message...]", stream=0x60300028e9d0) at ../../gdb/utils.c:1811
    ...

This comes from what appears to be a oversight in fputs_maybe_filtered.  This
function has a FILTER parameter which if true makes the function pause after
every screenful (i.e. triggers pagination).

The filter parameter is correctly used to guard the first place where
prompt_for_continue.  There is a second place in the function which can call
prompt_for_continue, but is currently unguarded.  I believe that this is an
oversight, this patch fixes that.

Tested on Linux-x86_64, no regression observed.

Change-Id: Iad8ffd50a87cf20077500878e2564b5a7dc81ece