illegal memory access in scroll_lastframe

[dankamongmen]

See #2875, especially the initial comment:

=42726== Thread 6:
==42726== Source and destination overlap in memcpy(0xe164190, 0xe164190, 1216)
==42726==    at 0x4856641: memcpy (vg_replace_strmem.c:1167)
==42726==    by 0x48CB419: scroll_lastframe (render.c:971)
==42726==    by 0x48CB419: ncpile_render (render.c:1570)
==42726==    by 0x20521F: notcurses_render (notcurses.h:1151)
==42726==    by 0x20521F: render (NotCurses.hh:205)
==42726==    by 0x20521F: Tick(ncpp::NotCurses*, unsigned long) (input.cpp:228)
==42726==    by 0x205309: Ticker(ncpp::NotCurses*) (input.cpp:238)
==42726==    by 0x20679E: __invoke<void (*)(ncpp::NotCurses *), ncpp::NotCurses *> (include/c++/v1/__type_traits/invoke.h:344)
==42726==    by 0x20679E: __thread_execute<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(ncpp::NotCurses *), ncpp::NotCurses *, 2UL> (include/c++/v1/__thread/thread.h:193)
==42726==    by 0x20679E: void* std::__1::__thread_proxy[abi:se180100]<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, void (*)(ncpp::NotCurses*), ncpp::NotCurses*> >(void*) (include/c++/v1/__thread/thread.h:202)
==42726==    by 0x48FAB04: ??? (in /lib/libthr.so.3)
==42726== 

this seems pretty easily reproducible using a small terminal window.

[dankamongmen]

do we simply need a memmove() here? that would eliminate the valgrind warning, but it might not actually be what we want to copy. let me look more closely...

[dankamongmen]

so we're doing this copy once per line in the scrolled region, and each memcpy() applies to a single line. so no, i don't think we want to be copying within a line, and thus memmove() is not a good fix. we probably simply shouldn't be hitting this line at all.

[dankamongmen]

ok yeah targy ought be preserved across all three sections of scroll_lastframe(), but instead we're resetting it in the second section. whoops!

[dankamongmen]

hrmmm, that doesn't seem to have fixed it.

[dankamongmen]

we have src == dst in the valgrind output. given that

srcidx = dstidx + rows * nc->lfdimx

this can only happen if rows == 0. this can only happen if scroll_lastframe() is called with 0. scroll_lastframe() is only called from ncpile_render(). i'm now guarding that with a 0 check.

[dankamongmen]

fix verified