bech32: fix overflow bug.

Copied from upstream:

 2b0aac650c

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/bech32.c

@@ -99,7 +99,7 @@ int bech32_decode(char* hrp, uint8_t *data, size_t *data_len, const char *input,
         ++(*data_len);
     }
     hrp_len = input_len - (1 + *data_len);
-    if (hrp_len < 1 || *data_len < 6) {
+    if (1 + *data_len >= input_len || *data_len < 6) {
         return 0;
     }
     *(data_len) -= 6;