leak in test_util_decompress_dos_impl()

On master, make check results in multiple errors of the form

=================================================================
==52419==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 3145728 byte(s) in 3 object(s) allocated from:
    #0 0x7f4b123de602 in malloc (/usr/lib/x86_64-linux-gnu/libasan.so.2+0x98602)
    #1 0x55a23af687ea in tor_malloc_ src/common/util.c:150
    #2 0x55a23af68891 in tor_malloc_zero_ src/common/util.c:178
    #3 0x55a23aa4a70a in test_util_decompress_dos_impl src/test/test_util.c:2555
    #4 0x55a23aa4a70a in test_util_decompress_dos src/test/test_util.c:2597
    #5 0x55a23aaa7468 in testcase_run_bare_ src/ext/tinytest.c:106
    #6 0x55a23aaa78a0 in testcase_run_one src/ext/tinytest.c:253
    #7 0x55a23aaa8dd3 in tinytest_main src/ext/tinytest.c:435
    #8 0x55a23a69ed98 in main src/test/testing_common.c:319
    #9 0x7f4b1003a82f in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)

SUMMARY: AddressSanitizer: 3145728 byte(s) leaked in 3 allocation(s).
FAIL src/test/test (exit status: 23)

It looks like a simple case of failing to free input; I'll write up a patch.