Avoid leak in check_outside_mappings.

[bernhardu]

When I tried to update my ASan branch I received this report in the mmap_shared_extern test, unfortunately missing the important lines, but with this patch the report disappears:

==460704==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 32816 byte(s) in 1 object(s) allocated from:
    #0 0x7f8d926f4c57 in malloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:69
    #1 0x7f8d91ff89c4 in __alloc_dir ../sysdeps/unix/sysv/linux/opendir.c:115

SUMMARY: AddressSanitizer: 32816 byte(s) leaked in 1 allocation(s).