Fix triggerable BUG() when decoding hsv3 descriptors.

Also improve the unittest to make sure it catches the right error.

changes/bug23233

@@ -0,0 +1,4 @@
+  o Minor bugfixes (hidden service):
+    - Fix a BUG alert during HSv3 descriptor decoding that could trigger with a
+      specially crafted descriptor. Fixes bug #23233; bugfix on 0.3.0.1-alpha.
+      Bug found by "haxxpop".

src/or/hs_descriptor.c

@@ -1852,7 +1852,8 @@ desc_sig_is_valid(const char *b64_sig,
   sig_start = tor_memstr(encoded_desc, encoded_len, "\n" str_signature);
   /* Getting here means the token parsing worked for the signature so if we
    * can't find the start of the signature, we have a code flow issue. */
-  if (BUG(!sig_start)) {
+  if (!sig_start) {
+    log_warn(LD_GENERAL, "Malformed signature line. Rejecting.");
     goto err;
   }
   /* Skip newline, it has to go in the signature check. */

src/test/test_hs_descriptor.c

@@ -569,8 +569,12 @@ test_decode_bad_signature(void *arg)
   /* Update approx time to dodge cert expiration */
   update_approx_time(1502661599);
 
+
+  setup_full_capture_of_logs(LOG_WARN);
   ret = hs_desc_decode_plaintext(HS_DESC_BAD_SIG, &desc_plaintext);
   tt_int_op(ret, OP_EQ, -1);
+  expect_log_msg_containing("Malformed signature line. Rejecting.");
+  teardown_capture_of_logs();
 
  done: ;
 }