Ensure reset of security provider preference. (#1013)

src/main/java/net/schmizz/sshj/common/SecurityUtils.java

@@ -276,6 +276,9 @@ public class SecurityUtils {
      */
     public static synchronized void setSecurityProvider(String securityProvider) {
         SecurityUtils.securityProvider = securityProvider;
+        if(null == securityProvider) {
+            SecurityUtils.registerBouncyCastle = null;
+        }
         registrationDone = false;
     }
 

src/test/java/net/schmizz/sshj/transport/kex/Curve25519DHTest.java

@@ -17,6 +17,7 @@ package net.schmizz.sshj.transport.kex;
 
 import net.schmizz.sshj.common.SecurityUtils;
 import net.schmizz.sshj.transport.random.JCERandom;
+import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
@@ -43,6 +44,7 @@ public class Curve25519DHTest {
     };
 
     @BeforeEach
+    @AfterEach
     public void clearSecurityProvider() {
         SecurityUtils.setSecurityProvider(null);
     }