Handle when print_paddr tries to print invalid strings

In the PunyInform libary there's an object, CheapScenery that lhave code like below:

default:
    #ifdef SceneryReply;
    if(SceneryReply ofclass string)
        print_ret (string) SceneryReply;
    i = location.&cheap_scenery;
    w1 = self.number;
    if(SceneryReply(i-->w1, i-->(w1 + 1)))
        rtrue;
    #endif;
    "No need to concern yourself with that.";

The SceneryReply can both be a string or a routine, which makes the compiler to compile this unreachable code print_ret (string) SceneryReply; in the case it is a routine. When TXD tries to decompile this it throw a message to stderr, Warning: printing of nonexistent string. The decompiled snippet looks like below:

Routine 43b4, 2 locals

 43b5:  41 f9 01 43             JE              Ge9,#01 [FALSE] 43ba
 43b9:  b1                      RFALSE
 43ba:  e0 07 42 6c 10 bf 04 00 CALL_VS         109b0 (#10bf,#04) -> -(SP)
 43c2:  a0 00 c7                JZ              (SP)+ [TRUE] 43ca
 43c5:  8d 10 bf                PRINT_PADDR     42fc
 43c8:  bb                      NEW_LINE
 43c9:  b0                      RTRUE
 43ca:  e0 23 41 f1 10 00 4a 01 CALL_VS         107c4 (G00,#004a) -> L00
 43d2:  51 fb 08 ff             GET_PROP        Geb,#08 -> Gef
 43d6:  2d 02 ff                STORE           L01,Gef
 43d9:  54 02 01 00             ADD             L01,#01 -> -(SP)
 43dd:  6f 01 00 00             LOADW           L00,(SP)+ -> -(SP)
 43e1:  6f 01 02 00             LOADW           L00,L01 -> -(SP)
 43e5:  e0 2b 10 bf 00 00 00    CALL_VS         42fc ((SP)+,(SP)+) -> -(SP)
 43ec:  a0 00 41                JZ              (SP)+ [FALSE] RTRUE
 43ef:  b3 ...                  PRINT_RET       "No need to concern yourself
with that."

It would be better if it printed print_paddr [invalid string: 42fc]

This can be done by changing this snippet in txd.c

    case STATIC:
	if (decode.first_pass == 0) {
	    addr = (unsigned long) code_scaler * value +
		   (unsigned long) story_scaler * header.strings_offset;
	    string = lookup_string (addr);
	    if (string != 0)
		tx_printf ("%c%03d", (option_inform) ? 's' : 'S', (int) string);
	    else {
#ifndef TXD_DEBUG
		(void) fprintf (stderr,
				"\nWarning: printing of nonexistent string\n");
#endif
		tx_printf ("%lx", addr);
	    }
	}

to

    case STATIC:
	if (decode.first_pass == 0) {
	    addr = (unsigned long) code_scaler * value +
		   (unsigned long) story_scaler * header.strings_offset;
	    string = lookup_string (addr);
	    if (string != 0)
		tx_printf ("%c%03d", (option_inform) ? 's' : 'S', (int) string);
	    else
		tx_printf ("[invalid string: %lx]", addr);
	}