Initialize Function::domain

Fixes uninitialized memory read in oss-fuzz 15395
diff --git a/poppler/Function.cc b/poppler/Function.cc
index e7973e2..6452539 100644
--- a/poppler/Function.cc
+++ b/poppler/Function.cc
@@ -48,7 +48,7 @@
 // Function
 //------------------------------------------------------------------------
 
-Function::Function() {
+Function::Function() : domain{} {
 }
 
 Function::~Function() {