py/mpz.c: Fix bug with shl not truncating zero digits correctly.
diff --git a/py/mpz.c b/py/mpz.c
index f54bde8..241fa79 100644
--- a/py/mpz.c
+++ b/py/mpz.c
@@ -97,7 +97,7 @@
 
     // work out length of result
     jlen += n_whole;
-    if (idig[jlen - 1] == 0) {
+    while (jlen != 0 && idig[jlen - 1] == 0) {
         jlen--;
     }