qstr, objstr: Make sure that valid hash != 0, treat 0 as "not computed".

This feature was proposed with initial hashing RFC, and is prerequisite for
seamless static str object definition.

py/objstr.c

@@ -1463,7 +1463,8 @@ bool mp_obj_str_equal(mp_obj_t s1, mp_obj_t s2) {
     } else {
         GET_STR_HASH(s1, h1);
         GET_STR_HASH(s2, h2);
-        if (h1 != h2) {
+        // If any of hashes is 0, it means it's not valid
+        if (h1 != 0 && h2 != 0 && h1 != h2) {
             return false;
         }
         GET_STR_DATA_LEN(s1, d1, l1);

py/qstr.c

@@ -35,7 +35,12 @@ machine_uint_t qstr_compute_hash(const byte *data, uint len) {
     for (const byte *top = data + len; data < top; data++) {
         hash = ((hash << 5) + hash) ^ (*data); // hash * 33 ^ data
     }
-    return hash & 0xffff;
+    hash &= 0xffff;
+    // Make sure that valid hash is never zero, zero means "hash not computed"
+    if (hash == 0) {
+        hash++;
+    }
+    return hash;
 }
 
 typedef struct _qstr_pool_t {