This patch seems to fix the problem (sorry I failed to include it earlier.)
Index: pgrep.c
===================================================================
--- pgrep.c (revision 7226)
+++ pgrep.c (working copy)
@@ -455,7 +455,7 @@
} else {
list[++matches].num = task.XXXID;
}
- if (matches == size) {
+ if (matches == size -1) {
size *= 2;
list = realloc (list,
size * sizeof (union el));
