G4: M06 — add return after oversize-frame state reset

Why: When handle_rx detects an oversize frame header (>300 bytes), it
resets state (header, inframe, frame_expected_size) and re-calls itself
on any remaining data. But when data is empty after the header, the
method fell through to the frame-assembly code with inframe=b"",
eventually dispatching an empty frame to reader.handle_rx. Currently
absorbed by F06's umbrella try/except, but a guaranteed crash if that
guard moves. Adding a bare return after the reset block prevents the
fallthrough in both TCP and serial transports.
Refs: Forensics report finding M06

src/meshcore/serial_cx.py

@@ -106,7 +106,7 @@ class SerialConnection:
                 self.frame_expected_size = 0
                 if len(data) > 0: # rerun handle_rx on remaining data
                     self.handle_rx(data)
-                    return
+                return  # nothing left to process after reset
 
         upbound = self.frame_expected_size - len(self.inframe)
         if len(data) < upbound:

src/meshcore/tcp_cx.py

@@ -96,7 +96,7 @@ class TCPConnection:
                 self.frame_expected_size = 0
                 if len(data) > 0: # rerun handle_rx on remaining data
                     self.handle_rx(data)
-                    return
+                return  # nothing left to process after reset
 
         upbound = self.frame_expected_size - len(self.inframe)
         if len(data) < upbound :