Fix setfieldval() when the field is not set yet (#4705)

Author: alxroyer-thales

scapy/packet.py

@@ -638,16 +638,22 @@ def setfieldval(self, attr, val):
         # Optimization:
         #   Try to avoid replacing a value by the same value,
         #   and avoid recursive cache invalidation by the way.
-        old_val = self.getfieldval(attr)  # type: Any
-        if (
-            # In case of packets, check the given packet reference differs from the previous one.
-            (val is old_val) if isinstance(val, Packet)
-            # In case of lists, let's take the new value whatever (no optimization).
-            else False if isinstance(val, list)
-            # In the general case, compare the values.
-            else (val == old_val)
-        ):
-            return
+        try:
+            old_val = self.getfieldval(attr)  # type: Any
+            if (
+                # In case of packets, check the given packet reference differs from the previous one.
+                (val is old_val) if isinstance(val, Packet)
+                # In case of lists, let's take the new value whatever (no optimization).
+                else False if isinstance(val, list)
+                # In the general case, compare the values.
+                else (val == old_val)
+            ):
+                return
+        except AttributeError:
+            # Field name can't be found (yet?).
+            # Let the execution go on, especially on the payload.
+            # An `AttributeError` may eventually be raised in case of a `NoPayload`.
+            pass
 
         if self.deprecated_fields and attr in self.deprecated_fields:
             attr = self._resolve_alias(attr)