py, unix: Allow to compile with -Wsign-compare.

See issue #699.
diff --git a/py/showbc.c b/py/showbc.c
index 7c55d8c..51a1671 100644
--- a/py/showbc.c
+++ b/py/showbc.c
@@ -81,7 +81,7 @@
 
     // bytecode prelude: arg names (as qstr objects)
     printf("arg names:");
-    for (int i = 0; i < n_total_args; i++) {
+    for (mp_uint_t i = 0; i < n_total_args; i++) {
         printf(" %s", qstr_str(MP_OBJ_QSTR_VALUE(*(mp_obj_t*)ip)));
         ip += sizeof(mp_obj_t);
     }
@@ -539,7 +539,7 @@
 
 void mp_bytecode_print2(const byte *ip, mp_uint_t len) {
     mp_showbc_code_start = ip;
-    while (ip - mp_showbc_code_start < len) {
+    while (ip < len + mp_showbc_code_start) {
         printf("%02u ", (uint)(ip - mp_showbc_code_start));
         ip = mp_bytecode_print_str(ip);
         printf("\n");