Merge #21364: fuzz: Avoid -Wreturn-type warnings

3f36468 fuzz: Avoid -Wreturn-type warnings (practicalswift)

Pull request description:

  Avoid `-Wreturn-type` warnings.

  Closes #21355.

ACKs for top commit:
  MarcoFalke:
    cr ACK 3f36468
  fanquake:
    ACK 3f36468 - thanks for cleaning this up.

Tree-SHA512: 6fa2640a26e64d2bea60e016ad14b5c434137fedc0b3bf2ac244f02f9b1cd303d1ebac4ac4e6791534560f8311c4cbe9395c2ce94d7ec022d3b192f1ea070809

src/test/fuzz/util.h

@@ -10,6 +10,7 @@
 #include <attributes.h>
 #include <chainparamsbase.h>
 #include <coins.h>
+#include <compat.h>
 #include <consensus/consensus.h>
 #include <merkleblock.h>
 #include <net.h>
@@ -545,11 +546,13 @@ class FuzzedSock : public Sock
     SOCKET Get() const override
     {
         assert(false && "Not implemented yet.");
+        return INVALID_SOCKET;
     }
 
     SOCKET Release() override
     {
         assert(false && "Not implemented yet.");
+        return INVALID_SOCKET;
     }
 
     void Reset() override