Fix warnings: ex_eval.c: report_pending(): Np dereference: FP.

Problem    : Dereference of null pointer @ 711.
Diagnostic : False positive.
Rationale  : Codepath producing error invokes this function with values
             `action=RPC_DISCARD, pending=CSTP_FINISH, value=NULL`.
             Now, for some reason, the analyzer is remembering that
             `value` is null, and that `action` is `RPC_DISCARD`, but
             it's not remembering that `pending` is `CSTP_FINISH`.
             Then, it's taking the wrong branch in the switch for
             `pending`. That path would never occur invocating the
             function with those values.
Resolution : Assert function precondition between `pending` and `value`.
             This is, let the compiler know that `value` being null
             implies `pending` not containing `CSTP_THROW`.

src/nvim/ex_eval.c

@@ -9,8 +9,8 @@
 /*
  * ex_eval.c: functions for Ex command line for the +eval feature.
  */
+#include <assert.h>
 #include <stdbool.h>
-
 #include <inttypes.h>
 
 #include "nvim/vim.h"
@@ -670,6 +670,7 @@ static void report_pending(int action, int pending, void *value)
   char        *s;
   int save_msg_silent;
 
+  assert(value || !(pending & CSTP_THROW));
 
   switch (action) {
   case RP_MAKE: