ubsan: Don’t call v[0] on empty vectors

util/mac/xattr.cc

 // Copyright 2014 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 23 matching lines @@
                                  0, 0, 0);
   if (buffer_size < 0) {
     if (errno == ENOATTR)
       return XattrStatus::kNoAttribute;
     PLOG(ERROR) << "getxattr size " << name << " on file " << file.value();
     return XattrStatus::kOtherError;
   }
 
   // Resize the buffer and read into it.
   value->resize(buffer_size);
-  ssize_t bytes_read = getxattr(file.value().c_str(), name.data(),
-                                &(*value)[0], value->size(),
-                                0, 0);
-  if (bytes_read < 0) {
-    PLOG(ERROR) << "getxattr " << name << " on file " << file.value();
-    return XattrStatus::kOtherError;
+  if (!value->empty()) {
+    ssize_t bytes_read = getxattr(file.value().c_str(), name.data(),
+                                  &(*value)[0], value->size(),
+                                  0, 0);
+    if (bytes_read < 0) {
+      PLOG(ERROR) << "getxattr " << name << " on file " << file.value();
+      return XattrStatus::kOtherError;
+    }
+    DCHECK_EQ(bytes_read, buffer_size);
   }
-  DCHECK_EQ(bytes_read, buffer_size);
 
   return XattrStatus::kOK;
 }
 
 bool WriteXattr(const base::FilePath& file,
                 const base::StringPiece& name,
                 const std::string& value) {
   int rv = setxattr(file.value().c_str(), name.data(), value.c_str(),
       value.length(), 0, 0);
   PLOG_IF(ERROR, rv != 0) << "setxattr " << name << " on file "
@@ skipping 77 matching lines @@
 }
 
 bool WriteXattrTimeT(const base::FilePath& file,
                      const base::StringPiece& name,
                      time_t value) {
   std::string tmp = base::StringPrintf("%ld", value);
   return WriteXattr(file, name, tmp);
 }
 
 }  // namespace crashpad