(CANT_REACH): New. Use it instead of CANT_HAPPEN() where possible.

(oops): Cope with NULL argument passed by CANT_REACH().

src/lib/commands/buil.c

@@ -273,7 +273,7 @@ buil(void)
 		built = build_plane(&sect, pp, sect.sct_item, tlev);
 		break;
 	    default:
-		CANT_HAPPEN("Bad WHAT");
+		CANT_REACH();
 		return RET_FAIL;
 	    }
 	    if (built) {

src/lib/commands/load.c

@@ -337,7 +337,7 @@ gift(natid givee, natid giver, void *ptr, int type, char *mesg)
 	p = prland(ptr);
 	break;
     default:
-	CANT_HAPPEN("bad TYPE");
+	CANT_REACH();
 	p = "a red herring";
     }
 

src/lib/commands/prod.c

@@ -296,7 +296,7 @@ prod(void)
 		pr(" %4.0f ", real);
 		break;
 	    default:
-		CANT_HAPPEN("bad TYPE");
+		CANT_REACH();
 		pr("  ??? ");
 		break;
 	    }

src/lib/commands/xdump.c

@@ -142,7 +142,7 @@ xdprval(struct valstr *val, char *sep)
 	    pr("%snil", sep);
 	break;
     default:
-	CANT_HAPPEN("Bad VAL type");
+	CANT_REACH();
 	pr("0");
     }
     return " ";