outobj: fix buffer overflow when too many segments in a group

The number of entries in a group was hard-coded, but not enforced.

Reported-by: <BreakingBad6@github.com>
Fixes: #203
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>

Author: Nick Clifton

output/outobj.c

@@ -1736,6 +1736,10 @@ obj_directive(enum directive directive, char *value)
                      */
                     continue;
                 }
+		if (grp->nentries >= GROUP_MAX) {
+                    nasm_nonfatal("too many segments in a group");
+                    return DIRR_ERROR;
+                }
                 for (seg = seghead; seg; seg = seg->next)
                     if (!strcmp(seg->name, p))
                         break;