[PATCH]: Fix failing test malloc/tst-interpose-nothread with GCC 7.

[Stefan Liebler]

Hi,

The test malloc/tst-interpose-nothread fails on s390x if built
with GCC 7 and recent glibc commit "Remove the str(n)dup inlines
from string/bits/string2.h. Although inlining"
(ae65d4f3c3995279ca458c460ebf8bab1885fa03) with output:
error: free: 0x3fffdffa010: invalid allocation index: 0 (not less than 0)

The destructor check_for_allocations in malloc/tst-interpose-aux.c is
called twice.  One time after the test-child-process has finished
successfully and once after the test-parent-process finishes.
During the latter invocation, allocation_index == 0.  GCC 7 is
now inlining the free function and calls unconditionally fail in
get_header as header->allocation_index (type == size_t) is always
 >= allocation_index (== 0).
Before the mentioned commit above, strdup was replaced by strlen,
malloc and memcpy.  The malloc call was also inlined and
allocation_index was set to one.

This patch moves the already existing compiler barrier before
the invocation of free.

Okay to commit?

ChangeLog:

	* malloc/tst-interpose-aux.c (check_for_allocations):
	Move compiler barrier before free.
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