write: fix reserving empty section error

if we copy a elf with a empty section and the sh_addralign is not 0,
it will return a unaligned offset.

Signed-off-by: lzwycc <lzw32321226@163.com>

src/write/elf/writer.rs

@@ -216,10 +216,9 @@ impl<'a> Writer<'a> {
     ///
     /// Returns the aligned offset of the start of the range.
     pub fn reserve(&mut self, len: usize, align_start: usize) -> usize {
-        if len == 0 {
-            return self.len;
+        if align_start > 1 {
+            self.len = util::align(self.len, align_start);
         }
-        self.len = util::align(self.len, align_start);
         let offset = self.len;
         self.len += len;
         offset