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

snapshot/mac/mach_o_image_segment_reader.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 77 matching lines @@
                                         segment_command_.vmaddr,
                                         segment_command_.vmsize);
   if (!segment_range.IsValid()) {
     LOG(WARNING) << base::StringPrintf("invalid segment range 0x%llx + 0x%llx",
                                        segment_command_.vmaddr,
                                        segment_command_.vmsize) << segment_info;
     return false;
   }
 
   sections_.resize(segment_command_.nsects);
-  if (!process_types::section::ReadArrayInto(
+  if (!sections_.empty() &&
+      !process_types::section::ReadArrayInto(
           process_reader,
           load_command_address + segment_command_.Size(),
           segment_command_.nsects,
           &sections_[0])) {
     LOG(WARNING) << "could not read sections" << segment_info;
     return false;
   }
 
   for (size_t section_index = 0;
        section_index < sections_.size();
@@ skipping 189 matching lines @@
 }
 
 void MachOImageSegmentReader::SetSlide(mach_vm_size_t slide) {
   INITIALIZATION_STATE_DCHECK_VALID(initialized_);
   INITIALIZATION_STATE_SET_INITIALIZING(initialized_slide_);
   slide_ = slide;
   INITIALIZATION_STATE_SET_VALID(initialized_slide_);
 }
 
 }  // namespace crashpad