cmd/guru: fix assignment to entry in nil map.

Fixes golang/go#14746.

Change-Id: Id79c43c7d3700b98c010aa7dc0aac2463164be09
Reviewed-on: https://go-review.googlesource.com/20988
Reviewed-by: Alan Donovan <adonovan@google.com>

cmd/guru/referrers.go

@@ -175,6 +175,9 @@ func globalReferrers(q *Query, qpkg, defpkg string, objposn token.Position, isPk
 	var users map[string]bool
 	if isPkgLevel {
 		users = rev[defpkg] // direct importers
+		if users == nil {
+			users = make(map[string]bool)
+		}
 		users[defpkg] = true // plus the defining package itself
 	} else {
 		users = rev.Search(defpkg) // transitive importers