bolt11: return error on unexpected lightning prefix

Remove the assertion so that an error is returned for invalid bech32.
An error is preferable to crashing the entire node if there's an extra
"lightning:" prefix:

  $ lightning-cli pay "lightning:lightning:"

Node log:
  pay: common/bolt11.c:718: bolt11_decode_nosig: Assertion `!has_lightning_prefix(str)' failed.
  pay: FATAL SIGNAL 6
  ...
  INFO    plugin-pay: Killing plugin: exited during normal operation
  **BROKEN** plugin-pay: Plugin marked as important, shutting down lightningd

common/bolt11.c

@@ -721,7 +721,6 @@ struct bolt11 *bolt11_decode_nosig(const tal_t *ctx, const char *str,
 	memset(have_field, 0, sizeof(have_field));
 	b11->routes = tal_arr(b11, struct route_info *, 0);
 
-	assert(!has_lightning_prefix(str));
 	if (strlen(str) < 8)
 		return decode_fail(b11, fail, "Bad bech32 string");
 

tests/fuzz/corpora/fuzz-bolt11/crash-98a2112c93362e35310c629081b5d60390062962

@@ -0,0 +1 @@
+lightning:

tests/fuzz/fuzz-bolt11.c

@@ -102,12 +102,6 @@ size_t LLVMFuzzerCustomMutator(uint8_t *fuzz_data, size_t size, size_t max_size,
 		return initial_input(fuzz_data, size, max_size);
 	}
 
-	// Strip (repeated) "lightning:" prefixes
-	while (strstarts(output, "lightning:") ||
-	       strstarts(output, "LIGHTNING:")) {
-		output = (char *)to_canonical_invstr(tmpctx, output);
-	}
-
 	// Write the result into `fuzz_data`.
 	size_t output_len = strlen(output);
 	if (output_len > max_size)