sd-bus: fix memstream buffer extraction

sd-bus: fix memstream buffer extraction

I'm getting the following error under valgrind:

==305970== Invalid free() / delete / delete[] / realloc()
==305970==    at 0x483E9F1: free (vg_replace_malloc.c:538)
==305970==    by 0x4012CD: mfree (alloc-util.h:48)
==305970==    by 0x4012EF: freep (alloc-util.h:83)
==305970==    by 0x4017F4: LLVMFuzzerTestOneInput (fuzz-bus-match.c:58)
==305970==    by 0x401A58: main (fuzz-main.c:39)
==305970==  Address 0x59972f0 is 0 bytes inside a block of size 8,192 free'd
==305970==    at 0x483FCE4: realloc (vg_replace_malloc.c:834)
==305970==    by 0x4C986F7: _IO_mem_finish (in /usr/lib64/libc-2.33.so)
==305970==    by 0x4C8F5E0: fclose@@GLIBC_2.2.5 (in /usr/lib64/libc-2.33.so)
==305970==    by 0x49D2CDB: fclose_nointr (fd-util.c:108)
==305970==    by 0x49D2D3D: safe_fclose (fd-util.c:124)
==305970==    by 0x4A4BCCC: fclosep (fd-util.h:41)
==305970==    by 0x4A4E00F: bus_match_to_string (bus-match.c:859)
==305970==    by 0x4016C2: LLVMFuzzerTestOneInput (fuzz-bus-match.c:58)
==305970==    by 0x401A58: main (fuzz-main.c:39)
==305970==  Block was alloc'd at
==305970==    at 0x483FAE5: calloc (vg_replace_malloc.c:760)
==305970==    by 0x4C98787: open_memstream (in /usr/lib64/libc-2.33.so)
==305970==    by 0x49D56D6: open_memstream_unlocked (fileio.c:97)
==305970==    by 0x4A4DEC5: bus_match_to_string (bus-match.c:859)
==305970==    by 0x4016C2: LLVMFuzzerTestOneInput (fuzz-bus-match.c:58)
==305970==    by 0x401A58: main (fuzz-main.c:39)
==305970==

So the fclose() which is called from _cleanup_fclose_ clearly reallocates the
buffer (maybe to save memory?). open_memstream(3) says:

  The locations referred to by these pointers are updated each time the
  stream is flushed (fflush(3)) and  when the stream is closed (fclose(3)).

This seems to mean that we should close the stream first before grabbing the
buffer pointer.