Fix warnings: ex_getln.c: init_history(): Double free: FP.

Problem    : Double free @ 4249.
Diagnostic : False positive.
Rationale  : Codepath leading to error contains two consecutive
             iterations in which `if (--j < 0)` is true.
             That executes `free` two consecutive times with the same
             value (hislen - 1) for j, with leads to double free.
             Now, that can only happen with j == 0 && hislen == 1.
             And that would imply j == hisidx[type] too, which would
             take the following break.
             So, the error codepath cannot really happen, but the
             compiler cannot deduce the last implication.
Resolution : We have two possible solutions for this:
             1.- Comparing value of j before and after updating it,
                 and breaking out of iteration if equal.
                 That changes nothing in functionality, but teaches the
                 compiler his proposed error codepath is impossible.
             2.- Nullify pointer after freeing.
                 This way, the compiler still thinks his error codepath
                 is possible, but it's not an error anymore, as
                 free(NULL) is a no-op.
             We opt for solution 2, as solution 1 requires adding
             logic that adds nothing (and having to explain that clearly
             in aside comments) just for the purpose of silencing
             warning. On the other hand, solution 2 improves the code,
             adding something considered good practice in any case,
             and therefore doesn't require further explanation.

src/nvim/ex_getln.c

@@ -4245,8 +4245,10 @@ void init_history(void)
           for (i = newlen - 1;; --i) {
             if (i >= 0)                         /* copy newest entries */
               temp[i] = history[type][j];
-            else                                /* remove older entries */
+            else {                              /* remove older entries */
               free(history[type][j].hisstr);
+              history[type][j].hisstr = NULL;
+            }
             if (--j < 0)
               j = hislen - 1;
             if (j == hisidx[type])