fix toBuffer for image/jpeg

Was mixing vec.capacity() and vec.size(). With MSVC the vec's growth factor is 1.5 (same as what I'm using), so this bug didn't manifest. With glibc it's 2.0. Also had an off-by-one.

Fixes Automattic#1350

src/closure.cc

@@ -4,15 +4,15 @@ void JpegClosure::init_destination(j_compress_ptr cinfo) {
   JpegClosure* closure = (JpegClosure*)cinfo->client_data;
   closure->vec.resize(PAGE_SIZE);
   closure->jpeg_dest_mgr->next_output_byte = &closure->vec[0];
-  closure->jpeg_dest_mgr->free_in_buffer = closure->vec.capacity();
+  closure->jpeg_dest_mgr->free_in_buffer = closure->vec.size();
 }
 
 boolean JpegClosure::empty_output_buffer(j_compress_ptr cinfo) {
   JpegClosure* closure = (JpegClosure*)cinfo->client_data;
-  size_t currentSize = closure->vec.capacity();
+  size_t currentSize = closure->vec.size();
   closure->vec.resize(currentSize * 1.5);
-  closure->jpeg_dest_mgr->next_output_byte = &closure->vec[currentSize - 1];
-  closure->jpeg_dest_mgr->free_in_buffer = closure->vec.capacity() - currentSize;
+  closure->jpeg_dest_mgr->next_output_byte = &closure->vec[currentSize];
+  closure->jpeg_dest_mgr->free_in_buffer = closure->vec.size() - currentSize;
   return true;
 }
 

src/closure.h

@@ -56,7 +56,7 @@ struct JpegClosure : Closure {
   uint32_t quality = 75;
   uint32_t chromaSubsampling = 2;
   bool progressive = false;
-  jpeg_destination_mgr* jpeg_dest_mgr;
+  jpeg_destination_mgr* jpeg_dest_mgr = nullptr;
 
   static void init_destination(j_compress_ptr cinfo);
   static boolean empty_output_buffer(j_compress_ptr cinfo);