[parform] Crash in #FactDollar about form HOT 1 OPEN

S x;
#$a = 1+x;
#factdollar $a
#message `$a[0]'
#message `$a[1]'
.end

==8363== Invalid read of size 8
==8363==    at 0x447FB0: WriteDollarFactorToBuffer (dollar.c:705)
==8363==    by 0x596328: OpenStream (tools.c:377)
==8363==    by 0x50E155: GetChar (pre.c:271)
==8363==    by 0x5102A4: LoadInstruction (pre.c:1109)
==8363==    by 0x50FFB6: PreProInstruction (pre.c:1029)
==8363==    by 0x50F8EC: PreProcessor (pre.c:857)
==8363==    by 0x5729A4: main (startup.c:1519)
==8363==  Address 0x0 is not stack'd, malloc'd or (recently) free'd