Fix invalid access of display line end

When a line only contains non-range atoms we can end-up accessing
past the end atom.

Add a test that shows the issue when run with valgrind, it is
unfortunately quite hard to trigger a crash because the invalidly
accessed byte usually leads to the correct code path being taken
(when != DisplayAtom::Range) so we have only 1 in 255 chance of
triggerring a crash.

Fixes #4927

src/highlighters.cc

@@ -1820,7 +1820,7 @@ struct ForwardHighlighterApplier
         {
             auto& line = *cur_line;
             auto first = std::find_if(cur_atom, line.end(), [&](auto&& atom) { return atom.has_buffer_range() and atom.end() > begin; });
-            if (first->type() == DisplayAtom::Range and first->begin() < begin)
+            if (first != line.end() and first->type() == DisplayAtom::Range and first->begin() < begin)
                 first = ++line.split(first, begin);
             auto idx = first - line.begin();
 

test/regression/4927-crash-jumping-to-eol/cmd

@@ -0,0 +1 @@
+<a-l>

test/regression/4927-crash-jumping-to-eol/in

@@ -0,0 +1,4 @@
+xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
+
+
+

test/regression/4927-crash-jumping-to-eol/rc

@@ -0,0 +1,3 @@
+add-highlighter global/ column 60 red
+add-highlighter buffer/regions regions
+add-highlighter buffer/regions/ default-region fill green