Added proper error about broken 0-values for enums.

Makefile

@@ -157,7 +157,7 @@ src/descriptor.pb: src/descriptor.proto
 
 descriptorgen: src/descriptor.pb src/upbc
 	@# Regenerate descriptor_const.h
-	./tools/upbc -o src/descriptor src/descriptor.pb
+	./src/upbc -o src/descriptor src/descriptor.pb
 	cd src && xxd -i descriptor.pb > descriptor.c
 
 src/upbc: src/upbc.c $(LIBUPB)

src/upb_def.c

@@ -476,6 +476,12 @@ static upb_flow_t upb_enumdef_EnumValueDescriptorProto_value(void *_b,
       break;
     case GOOGLE_PROTOBUF_ENUMVALUEDESCRIPTORPROTO_NUMBER_FIELDNUM:
       b->number = upb_value_getint32(val);
+      if (b->number == 0) {
+        upb_seterr(&b->status, UPB_ERROR,
+                   "Enums with a value of 0 are currently broken; see "
+                   "http://code.google.com/p/upb/issues/detail?id=1.");
+        return UPB_BREAK;
+      }
       b->saw_number = true;
       break;
     default: