fix(context): index out of range in Context::Highlight (#867)

clamp new_index between 0 and candidate_count - 1

---------

Co-authored-by: 居戎氏 <chen.sst@gmail.com>

src/rime/context.cc

@@ -4,6 +4,7 @@
 //
 // 2011-05-08 GONG Chen <chen.sst@gmail.com>
 //
+#include <algorithm>
 #include <utility>
 #include <rime/candidate.h>
 #include <rime/context.h>
@@ -127,12 +128,9 @@ bool Context::Highlight(size_t index) {
   if (composition_.empty() || !composition_.back().menu)
     return false;
   Segment& seg(composition_.back());
-  size_t new_index = index;
   size_t candidate_count = seg.menu->Prepare(index + 1);
-  if (index >= candidate_count) {
-    DLOG(INFO) << "selection index exceeds candidate pool, fallback to last";
-    new_index = candidate_count - 1;
-  }
+  size_t new_index =
+      candidate_count > 0 ? (std::min)(candidate_count - 1, index) : 0;
   size_t previous_index = seg.selected_index;
   if (previous_index == new_index) {
     DLOG(INFO) << "selection has not changed, currently at " << new_index;