erasure: ReadFile should honor proper offsets. (#1542)

Fixes #1535

xl-erasure-v1-readfile.go

@@ -174,13 +174,14 @@ func (xl XL) ReadFile(volume, path string, startOffset int64) (io.ReadCloser, er
 
 			// Verify if the offset is right for the block, if not move to
 			// the next block.
+
 			if startOffset > 0 {
 				startOffset = startOffset - int64(len(dataBlocks))
-				if startOffset > int64(len(dataBlocks)) {
+				// Start offset is greater than or equal to zero, skip the dataBlocks.
+				if startOffset >= 0 {
 					continue
 				}
-				// Fetch back the overflow offset, to skip from the current data
-				// blocks.
+				// Now get back the remaining offset if startOffset is negative.
 				startOffset = startOffset + int64(len(dataBlocks))
 			}