fs/binfmt_elf.c: don't free interpreter's ELF pheaders on common path

Static executables don't need to free NULL pointer.

It doesn't matter really because static executable is not common scenario
but do it anyway out of pedantry.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200219185330.GA4933@avx2


Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>