Explicitly close all AutoFiles that have been written

There is no way to report a close error from `AutoFile` destructor.
Such an error could be serious if the file has been written to because
it may mean the file is now corrupted (same as if write fails).

So, change all users of `AutoFile` that use it to write data to
explicitly close the file and handle a possible error.

src/test/fuzz/autofile.cpp

@@ -47,7 +47,7 @@ FUZZ_TARGET(autofile)
                 }
             },
             [&] {
-                auto_file.fclose();
+                (void)auto_file.fclose();
             },
             [&] {
                 ReadFromStream(fuzzed_data_provider, auto_file);
@@ -62,5 +62,7 @@ FUZZ_TARGET(autofile)
         if (f != nullptr) {
             fclose(f);
         }
+    } else {
+        (void)auto_file.fclose();
     }
 }

src/test/fuzz/policy_estimator.cpp

@@ -111,5 +111,6 @@ FUZZ_TARGET(policy_estimator, .init = initialize_policy_estimator)
         AutoFile fuzzed_auto_file{fuzzed_file_provider.open()};
         block_policy_estimator.Write(fuzzed_auto_file);
         block_policy_estimator.Read(fuzzed_auto_file);
+        (void)fuzzed_auto_file.fclose();
     }
 }

src/test/fuzz/policy_estimator_io.cpp

@@ -32,4 +32,5 @@ FUZZ_TARGET(policy_estimator_io, .init = initialize_policy_estimator_io)
     if (block_policy_estimator.Read(fuzzed_auto_file)) {
         block_policy_estimator.Write(fuzzed_auto_file);
     }
+    (void)fuzzed_auto_file.fclose();
 }

src/test/fuzz/utxo_snapshot.cpp

@@ -150,6 +150,7 @@ void utxo_snapshot_fuzz(FuzzBufferType buffer)
             WriteCompactSize(outfile, 999); // index of coin
             outfile << Coin{coinbase->vout[0], /*nHeightIn=*/999, /*fCoinBaseIn=*/0};
         }
+        assert(outfile.fclose() == 0);
     }
 
     const auto ActivateFuzzedSnapshot{[&] {