Remove C99 struct initialize to avoid warning
Remove C99 struct initialize to avoid warning

Initializing a struct with zeroes using {0} is legal in C99, but cause
a warning in Clang when the first member of the struct isn't a scalar
variable:

gpfdist.c:1299:44: warning: suggest braces around initialization of subobject
                   [-Wmissing-braces]
        struct fstream_filename_and_offset fos = {0};
                                                  ^
                                                  {}
This is a bug in Clang, but we also don't want to turn off this class
of warnings due to a this trivial false positive, as that might hide
actual warnings somewhere. Since the struct in question is guaranteed
to be set before being read, we can avoid the explicit initialization.

ashwinstar/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,50wu/gpdb,adam8157/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,jmcatamney/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,jmcatamney/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,lisakowen/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,lisakowen/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb,jmcatamney/gpdb,xinzweb/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,adam8157/gpdb,ashwinstar/gpdb,xinzweb/gpdb,50wu/gpdb,jmcatamney/gpdb,ashwinstar/gpdb