Fix an unintended sign extension

Reported by coverity:
>>>     CID 68353:  Unintended sign extension  (SIGN_EXTENSION)
>>>     Suspicious implicit sign extension: "file_header.e_shnum"
with type "unsigned short" (16 bits, unsigned) is promoted in
"file_header.e_shnum * file_header.e_shentsize" to type "int" (32
bits, signed), then sign-extended to type "unsigned long" (64 bits,
unsigned).  If "file_header.e_shnum * file_header.e_shentsize" is
greater than 0x7FFFFFFF, the upper bits of the result will all be 1.
Remove the redundant "/payload" parsing