objcopy: check input flavor before setting PE/COFF section alignment

coff_section_data() and elf_section_data() use the same underlying
field. The pointer being non-NULL therefore isn't sufficient to know
that pei_section_data() can validly be used on the incoming object.
Apparently in 64-bit-host builds the resulting memory corruption is
benign, whereas in 32-bit-host builds a segmentation fault occurs upon
de-referencing pei_section_data()'s return value.