[Groff] tbl bug with nospaces option

[Louis Guillaume]

Hi,

I just noticed that tbl was crashing under certain conditions with the 
"nospaces" option. Here's what I discovered:

Take a simple table like this:

.TS H
expand nospaces tab(@);
l c r.
\fBone @ two @ three\fP
.TH
1 @  2   @ 3
.TE

Everything works here. But, if, say we didn't have a value in the second 
column, but still had spaces, we get a segmentation fault in tbl:


.TS H
expand nospaces tab(@);
l c r.
\fBone @ two @ three\fP
.TH
1 @     @ 3
.TE


$ tbl se_test.ms
.if !\n(.g .ab GNU tbl requires GNU troff.
.if !dTS .ds TS
.if !dTE .ds TE
.lf 1 se_test.ms
.TS H
Segmentation fault

Removing the spaces, leaving "1 @@ 3", solves the problem. But seems 
like a bug in tbl to me...

I've reproduced this on OS X and NetBSD.

Louis