Updated doc for BN_clear, BN_CTX_end when param is NULL

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8532)

(cherry picked from commit 138ef774fe)

doc/man3/BN_CTX_start.pod

@@ -27,6 +27,7 @@ calls must be made before calling any other functions that use the
 B<ctx> as an argument.
 
 Finally, BN_CTX_end() must be called before returning from the function.
+If B<ctx> is NULL, nothing is done.
 When BN_CTX_end() is called, the B<BIGNUM> pointers obtained from
 BN_CTX_get() become invalid.
 

doc/man3/BN_new.pod

@@ -27,6 +27,7 @@ OPENSSL_secure_malloc(3) is used to store the value.
 BN_clear() is used to destroy sensitive data such as keys when they
 are no longer needed. It erases the memory used by B<a> and sets it
 to the value 0.
+If B<a> is NULL, nothing is done.
 
 BN_free() frees the components of the B<BIGNUM>, and if it was created
 by BN_new(), also the structure itself. BN_clear_free() additionally