fix NullReference in KeyExchangeECDH

for some reason d24fe20 introduced
its own names here which (as far as I can see) were never part
of the original BouncyCastle.

Switched to the correct names.

Author: mus65

src/Renci.SshNet/Security/KeyExchangeECDH256.cs

@@ -21,7 +21,7 @@ protected override X9ECParameters CurveParameter
         {
             get
             {
-                return SecNamedCurves.GetByName("P-256");
+                return SecNamedCurves.GetByName("secp256r1");
             }
         }
 

src/Renci.SshNet/Security/KeyExchangeECDH384.cs

@@ -21,7 +21,7 @@ protected override X9ECParameters CurveParameter
         {
             get
             {
-                return SecNamedCurves.GetByName("P-384");
+                return SecNamedCurves.GetByName("secp384r1");
             }
         }
 

src/Renci.SshNet/Security/KeyExchangeECDH521.cs

@@ -21,7 +21,7 @@ protected override X9ECParameters CurveParameter
         {
             get
             {
-                return SecNamedCurves.GetByName("P-521");
+                return SecNamedCurves.GetByName("secp521r1");
             }
         }