Add mbedtls_ecjpake_check(), tells if set up

This will be used in SSL to avoid the computation-heavy processing of EC
J-PAKE hello extensions in case we don't have an EC J-PAKE password

library/ecjpake.c

@@ -119,6 +119,21 @@ cleanup:
     return( ret );
 }
 
+/*
+ * Check if context is ready for use
+ */
+int mbedtls_ecjpake_check( const mbedtls_ecjpake_context *ctx )
+{
+    if( ctx->md_info == NULL ||
+        ctx->grp.id == MBEDTLS_ECP_DP_NONE ||
+        ctx->s.p == NULL )
+    {
+        return( MBEDTLS_ERR_ECP_BAD_INPUT_DATA );
+    }
+
+    return( 0 );
+}
+
 /*
  * Write a point plus its length to a buffer
  */