free NULL cleanup

Start ensuring all OpenSSL "free" routines allow NULL, and remove
any if check before calling them.
This gets DH_free, DSA_free, RSA_free

Reviewed-by: Matt Caswell <matt@openssl.org>

doc/crypto/DH_new.pod

@@ -18,6 +18,7 @@ DH_new() allocates and initializes a B<DH> structure.
 
 DH_free() frees the B<DH> structure and its components. The values are
 erased before the memory is returned to the system.
+If B<dh> is NULL nothing is done.
 
 =head1 RETURN VALUES
 

doc/crypto/DSA_new.pod

@@ -19,6 +19,7 @@ calling DSA_new_method(NULL).
 
 DSA_free() frees the B<DSA> structure and its components. The values are
 erased before the memory is returned to the system.
+If B<dsa> is NULL nothing is done.
 
 =head1 RETURN VALUES
 

doc/crypto/RSA_new.pod

@@ -19,6 +19,7 @@ calling RSA_new_method(NULL).
 
 RSA_free() frees the B<RSA> structure and its components. The key is
 erased before the memory is returned to the system.
+If B<rsa> is NULL nothing is done.
 
 =head1 RETURN VALUES