curses static analyzer bits

This is mostly just adding some Null guards ahead of
code that was already dereferencing pointers, so there
should be no change in behavior.

Also adds one validation of an array index that was
drawing a complaint.

win/curses/cursmisc.c

@@ -397,6 +397,7 @@ curses_str_remainder(const char *str, int width, int line_num)
         if (last_space == 0) {  /* No spaces found */
             last_space = count - 1;
         }
+        assert(IndexOk(last_space, substr));
         if (substr[last_space] == '\0') {
             break;
         }