Fixed UBSAN error.

.bazelrc

@@ -46,3 +46,12 @@ build:msan-libfuzzer --action_env=CXX=clang++
 build:msan-libfuzzer --@rules_fuzzing//fuzzing:cc_engine=@rules_fuzzing//fuzzing/engines:libfuzzer
 build:msan-libfuzzer --@rules_fuzzing//fuzzing:cc_engine_instrumentation=libfuzzer
 build:msan-libfuzzer --@rules_fuzzing//fuzzing:cc_engine_sanitizer=msan
+
+# --config=ubsan-libfuzzer
+build:ubsan-libfuzzer --action_env=CC=clang
+build:ubsan-libfuzzer --action_env=CXX=clang++
+build:ubsan-libfuzzer --@rules_fuzzing//fuzzing:cc_engine=@rules_fuzzing//fuzzing/engines:libfuzzer
+build:ubsan-libfuzzer --@rules_fuzzing//fuzzing:cc_engine_instrumentation=libfuzzer
+build:ubsan-libfuzzer --copt=-fsanitize=undefined
+build:ubsan-libfuzzer --linkopt=-fsanitize=undefined
+build:ubsan-libfuzzer --linkopt=-fsanitize-link-c++-runtime

upb/decode.c

@@ -918,7 +918,7 @@ static const char *decode_unknown(upb_decstate *d, const char *ptr,
     }
 
     assert(start == d->debug_valstart);
-    uint32_t tag = (field_number << 3) | wire_type;
+    uint32_t tag = ((uint32_t)field_number << 3) | wire_type;
     start = decode_reverse_skip_varint(start, tag);
     assert(start == d->debug_tagstart);