Fixed a crash bug in stb_textedit.h (#681)

stb_textedit.h

@@ -1,4 +1,5 @@
 // [ImGui] this is a slightly modified version of stb_truetype.h 1.8
+// [ImGui] - fixed a crash bug in stb_textedit_discard_redo (#681)
 // [ImGui] - fixed some minor warnings
 // [ImGui] - added STB_TEXTEDIT_MOVEWORDLEFT/STB_TEXTEDIT_MOVEWORDRIGHT custom handler (#473)
 
@@ -1095,7 +1096,7 @@ static void stb_textedit_discard_redo(StbUndoState *state)
          int n = state->undo_rec[k].insert_length, i;
          // delete n characters from all other records
          state->redo_char_point = state->redo_char_point + (short) n; // vsnet05
-         STB_TEXTEDIT_memmove(state->undo_char + state->redo_char_point, state->undo_char + state->redo_char_point-n, (size_t) ((size_t)(STB_TEXTEDIT_UNDOSTATECOUNT - state->redo_char_point)*sizeof(STB_TEXTEDIT_CHARTYPE)));
+         STB_TEXTEDIT_memmove(state->undo_char + state->redo_char_point, state->undo_char + state->redo_char_point-n, (size_t) ((size_t)(STB_TEXTEDIT_UNDOCHARCOUNT - state->redo_char_point)*sizeof(STB_TEXTEDIT_CHARTYPE)));
          for (i=state->redo_point; i < k; ++i)
             if (state->undo_rec[i].char_storage >= 0)
                state->undo_rec[i].char_storage = state->undo_rec[i].char_storage + (short) n; // vsnet05