lib/mpi: mpi_write_sgl(): fix style issue with lzero decrement

Within the copying loop in mpi_write_sgl(), we have

  if (lzeros > 0) {
    ...
    lzeros -= sizeof(alimb);
  }

However, at this point, lzeros < sizeof(alimb) holds. Make this fact
explicit by rewriting the above to

  if (lzeros) {
    ...
    lzeros = 0;
  }
Signed-off-by: NNicolai Stange <nicstange@gmail.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

lib/mpi/mpicoder.c

@@ -402,14 +402,14 @@ int mpi_write_to_sgl(MPI a, struct scatterlist *sgl, unsigned *nbytes,
 #else
 #error please implement for this limb size.
 #endif
-		if (lzeros > 0) {
+		if (lzeros) {
 			mpi_limb_t *limb1 = (void *)p - sizeof(alimb);
 			mpi_limb_t *limb2 = (void *)p - sizeof(alimb)
 				+ lzeros;
 			*limb1 = *limb2;
 			p -= lzeros;
 			y = lzeros;
-			lzeros -= sizeof(alimb);
+			lzeros = 0;
 		}
 
 		p = p - (sizeof(alimb) - y);