py/qstr: Special case qstr_find_strn for empty string.

This handles the case where an empty bytes/bytearray/str could pass in
NULL as the str argument (with length zero). This would result in UB in
strncmp. Even though our bare-metal implementation of strncmp handles
this, best to avoid it for when we're using system strncmp.

This work was funded through GitHub Sponsors.
Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

py/qstr.c

@@ -233,6 +233,11 @@ STATIC qstr qstr_add(mp_uint_t hash, mp_uint_t len, const char *q_ptr) {
 }
 
 qstr qstr_find_strn(const char *str, size_t str_len) {
+    if (str_len == 0) {
+        // strncmp behaviour is undefined for str==NULL.
+        return MP_QSTR_;
+    }
+
     // work out hash of str
     size_t str_hash = qstr_compute_hash((const byte *)str, str_len);