py/qstr: Evaluate find_qstr only once then pass to Q_GET_HASH macro.

Q_GET_HASH may evaluate its argument more than once.

py/qstr.c

@@ -251,7 +251,8 @@ qstr qstr_from_strn(const char *str, size_t len) {
 }
 
 mp_uint_t qstr_hash(qstr q) {
-    return Q_GET_HASH(find_qstr(q));
+    const byte *qd = find_qstr(q);
+    return Q_GET_HASH(qd);
 }
 
 size_t qstr_len(qstr q) {