Fix warnings: eval.c: do_return(): Np dereference: FP.

Problem    : Dereference of null pointer @ 18841.
Diagnostic : False positive.
Rationale  : Suggested error path takes `reanimate` branch at 18827,
             assigning `rettv = current_funccal->rettv`. Then,
             inmediately after, it supposes rettv is null, which cannot
             happen, since current_funccal->rettv should always be non
             null.
Resolution : Assert current_funccal->rettv non null.

src/nvim/eval.c

@@ -18901,8 +18901,10 @@ int do_return(exarg_T *eap, int reanimate, int is_cmd, void *rettv)
     else {
       /* When undoing a return in order to make it pending, get the stored
        * return rettv. */
-      if (reanimate)
+      if (reanimate) {
+        assert(current_funccal->rettv);
         rettv = current_funccal->rettv;
+      }
 
       if (rettv != NULL) {
         /* Store the value of the pending return. */