Fix documentation for mbedtls_gcm_finish()

Fix implementation and documentation missmatch for the function
arguments to mbedtls_gcm_finish(). Also, removed redundant if condition
that always evaluates to true.

ChangeLog

@@ -45,6 +45,8 @@ Bugfix
    * Fix mbedtls_x509_get_sig() to update the ASN1 type in the mbedtls_x509_buf
      data structure until after error checks are successful. Found by
      subramanyam-c.
+   * Fix documentation and implementation missmatch for function arguments of
+     mbedtls_gcm_finish(). Found by cmiatpaar.
 
 Changes
    * Extended test coverage of special cases, and added new timing test suite.

include/mbedtls/gcm.h

@@ -190,8 +190,8 @@ int mbedtls_gcm_update( mbedtls_gcm_context *ctx,
  *                  16 bytes.
  *
  * \param ctx       GCM context
- * \param tag       buffer for holding the tag (may be NULL if tag_len is 0)
- * \param tag_len   length of the tag to generate
+ * \param tag       buffer for holding the tag
+ * \param tag_len   length of the tag to generate (must be at least 4)
  *
  * \return          0 if successful or MBEDTLS_ERR_GCM_BAD_INPUT
  */

library/gcm.c

@@ -415,7 +415,6 @@ int mbedtls_gcm_finish( mbedtls_gcm_context *ctx,
     if( tag_len > 16 || tag_len < 4 )
         return( MBEDTLS_ERR_GCM_BAD_INPUT );
 
-    if( tag_len != 0 )
     memcpy( tag, ctx->base_ectr, tag_len );
 
     if( orig_len || orig_add_len )