fuzz: avoid FuzzedSock::Recv() repeated errors with EAGAIN

If `recv(2)` returns an error (`-1`) and sets `errno` to a temporary
error like `EAGAIN` a proper application code is expected to retry the
operation.

If the fuzz data is exhausted, then `FuzzedSock::Recv()` will keep
returning `-1` and setting `errno` to the first element of
`recv_errnos[]` which happened to be `EAGAIN`. This may continue forever
or cause the fuzz test to run for a long time before some higher level
application "receive timeout" is triggered.

Thus, put `ECONNREFUSED` as first element of `recv_errnos[]`.

Author: vasild

src/test/fuzz/util.h

@@ -622,10 +622,13 @@ class FuzzedSock : public Sock
 
     ssize_t Recv(void* buf, size_t len, int flags) const override
     {
+        // Have a permanent error at recv_errnos[0] because when the fuzzed data is exhausted
+        // SetFuzzedErrNo() will always return the first element and we want to avoid Recv()
+        // returning -1 and setting errno to EAGAIN repeatedly.
         constexpr std::array recv_errnos{
+            ECONNREFUSED,
             EAGAIN,
             EBADF,
-            ECONNREFUSED,
             EFAULT,
             EINTR,
             EINVAL,