kepub: Fixed FD leak in content transformation

The FDs didn't get closed until after all content files were
processed due to the deferred close being in a for loop. This
wasn't noticed due to it not being an issue until the concurrency
in cbb9af0 causing it to exceed
the default FD limit on macOS for a large (500+ content file)
book.

I've ensured there weren't any other leaks left behind by testing
this with a batch conversion of large books and a ulimit of the
theoretical maximum amount kepubify should have:

    5 + max(1, max(cpu_cores, cpu_cores*max(3, cpu_cores + 1))
    |       |      |          |             |  ^ kepub (transformAllContentParallel): parallel content file transformation (+1 for scanning)
    |       |      |          |             ^ kepub (ConvertEPUB): packing/unpacking (zip+input_output+temp)
    |       |      |          ^ cmd/kepubify (convert): concurrent conversion
    |       |      ^ cmd/kepubify (convert): concurrent scanning/copying
    |       ^ cmd/kepubify (transform): path transformation
    ^ cmd/kepubify: kepubify itself (binary/stdout/stderr/etc...)

This issue was reported by email on 2020-01-01.

kepub/converter.go

@@ -136,14 +136,13 @@ func (c *Converter) transformAllContentParallel(dir string) error {
 	for i := 0; i < runtime.NumCPU(); i++ {
 		g.Go(func() error {
 			for fn := range contentFiles {
-				f, err := os.OpenFile(fn, os.O_RDWR, 0)
-				if err != nil {
+				if f, err := os.OpenFile(fn, os.O_RDWR, 0); err != nil {
 					return fmt.Errorf("open content file %#v: %w", fn, err)
-				}
-				defer f.Close()
-
-				if err := c.TransformContentDocFile(f); err != nil {
+				} else if err := c.TransformContentDocFile(f); err != nil {
+					f.Close()
 					return fmt.Errorf("transform content file %#v: %w", fn, err)
+				} else {
+					f.Close()
 				}
 
 				select {