newsym() impossible() band-aid

Prevent the main vision_recalc loop from triggering an impossible for
bad coordinates from newsym().  This hack may mean that the underlying
problem will remain unsolved but at least newsym won't cause the fuzzer
to panic or frighten players with impossible warnings.

src/vision.c

@@ -822,7 +822,18 @@ vision_recalc(int control)
                 if ((old_row[col] & IN_SIGHT)
                     || ((next_row[col] & COULD_SEE)
                         ^ (old_row[col] & COULD_SEE)))
+                {
+                    /*
+                     * TEMPORARY?  Sometimes we get here with col==0 and
+                     * newsym()'s impossible() for !isok() is being
+                     * triggered, so avoid calling it for <0,y>; other bad
+                     * coordinates will produce a panic() as they should.
+                     */
+                    if (col != 0)
+                    /*
+                     */
                     newsym(col, row);
+                }
             }
 
         } /* end for col . . */