OOM in setup_group

We alloc, seek and read using section sizes in object files.  Fuzzed
objects can have silly sizes, but that's OK if the system supports
memory over-commit.  The read fails because we hit EOF and that
usually results in a graceful exit.

But if we memset before the read then the invalid size results in
attempting to write to a huge number of memory pages, and an eventual
Out Of Memory after probably swapping like crazy.  So don't memset.
There really isn't a need to clear the section contents anyway.  All
bytes are written with a good object file by the read and following
loop converting section index in target order to ELF section header
pointer, and the only untidy bytes are the 4 bytes past the group
flags when pointers are 8 bytes.  Those don't matter but the patch
clears them for anyone poking around in a debugger.  On error paths
it's as good to free section contents as it is to clear them.

Noticed when looking at PR4110 fourth test case.

	PR 4110
	* elf.c (setup_group): Don't clear entire section contents,
	just the padding after group flags.  Release alloc'd memory
	after a seek or read failure.

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