Fixed 64-bit alignment panic in chunkedContentCoder on 32-bit platfor…

…ms. (#148)

Following the fix in #147, a 'panic: unaligned 64-bit atomic operation' is
still occurring on some platforms.

As noted in #147, this commit implements the alternative fix by placing the
bytesWritten field at the top of the struct. Per the Golang sync/atomic
documentation, on ARM, 386 and 32-bit MIPS systems, this is the reliable
way to prevent these issues.

Signed-off-by: Callum Jones <cj@icj.me>

contentcoder.go

@@ -37,6 +37,8 @@ var (
 )
 
 type chunkedContentCoder struct {
+	bytesWritten uint64 // atomic access to this variable, moved to top to correct alignment issues on ARM, 386 and 32-bit MIPS.
+
 	final     []byte
 	chunkSize uint64
 	currChunk uint64
@@ -45,7 +47,6 @@ type chunkedContentCoder struct {
 	compressed []byte // temp buf for snappy compression
 
 	w                io.Writer
-	bytesWritten     uint64 // atomic access to this variable
 	progressiveWrite bool
 
 	chunkMeta    []MetaData