fix(coverity): 353827 unnecessary null check #19209

Previous checks make sure the index is not out of bounds. Therefore jmp
is never NULL.

src/nvim/mark.c

@@ -280,7 +280,7 @@ fmark_T *get_jumplist(win_T *win, int count)
     }
     break;
   }
-  return jmp != NULL ? &jmp->fmark : NULL;
+  return &jmp->fmark;
 }
 
 /// Get mark in "count" position in the |changelist| relative to the current index.