pgsql: Doh --- what's really happening on buildfarm member grebe is that

Log Message:
-----------
Doh --- what's really happening on buildfarm member grebe is that its
malloc returns NULL for malloc(0). Defend against that case.

Modified Files:
--------------
pgsql/src/backend/utils/adt:
regexp.c (r1.74 -> r1.75)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/regexp.c?r1=1.74&r2=1.75)