write/elf: don't write empty strtab

If there were no strings, we were writing a strtab section header,
but not writing any data for the section.

src/write/elf/object.rs

@@ -598,10 +598,12 @@ impl<'a> Object<'a> {
         writer.reserve_symtab();
         if writer.symtab_shndx_needed() {
             writer.reserve_symtab_shndx_section_index();
+            writer.reserve_symtab_shndx();
+        }
+        if writer.strtab_needed() {
+            writer.reserve_strtab_section_index();
+            writer.reserve_strtab();
         }
-        writer.reserve_symtab_shndx();
-        writer.reserve_strtab_section_index();
-        writer.reserve_strtab();
 
         // Calculate size of relocations.
         for (index, section) in self.sections.iter().enumerate() {