Fix crash in broken documents

Fixes bug #101525
diff --git a/poppler/JBIG2Stream.cc b/poppler/JBIG2Stream.cc
index 067fb7f..4853588 100644
--- a/poppler/JBIG2Stream.cc
+++ b/poppler/JBIG2Stream.cc
@@ -760,6 +760,10 @@
   JBIG2Bitmap *slice;
   Guint xx, yy;
 
+  if (!data) {
+      return nullptr;
+  }
+
   slice = new JBIG2Bitmap(0, wA, hA);
   if (slice->isOk()) {
     slice->clearToZero();
@@ -3827,6 +3831,10 @@
   JBIG2BitmapPtr tpgrCXPtr2 = {0};
   int x, y, pix;
 
+  if (!refBitmap) {
+      return nullptr;
+  }
+
   bitmap = new JBIG2Bitmap(0, w, h);
   if (!bitmap->isOk())
   {