ecmult: Document and test ng=NULL in ecmult

src/ecmult.h

@@ -40,7 +40,10 @@
 /** The number of entries a table with precomputed multiples needs to have. */
 #define ECMULT_TABLE_SIZE(w) ((size_t)1 << ((w)-2))
 
-/** Double multiply: R = na*A + ng*G */
+/** Double multiply: R = na*A + ng*G
+ *
+ * Passing NULL as ng is equivalent to the zero scalar but a tiny bit faster.
+ */
 static void secp256k1_ecmult(secp256k1_gej *r, const secp256k1_gej *a, const secp256k1_scalar *na, const secp256k1_scalar *ng);
 
 typedef int (secp256k1_ecmult_multi_callback)(secp256k1_scalar *sc, secp256k1_ge *pt, size_t idx, void *data);