object-file: fix mmap() leak in odb_source_loose_read_object_stream()

We mmap() a loose object file, storing the result in the local variable
"mapped", which is eventually assigned into our stream struct as
"st.mapped". If we hit an error, we jump to an error label which does:

  munmap(st.mapped, st.mapsize);

to clean up. But this is wrong; we don't assign st.mapped until the end
of the function, after all of the "goto error" jumps. So this munmap()
is never cleaning up anything (st.mapped is always NULL, because we
initialize the struct with calloc).

Instead, we should feed the local variable to munmap().

This leak is due to 595296e124 (streaming: allocate stream inside the
backend-specific logic, 2025-11-23), which introduced the local
variable. Before that, we assigned the mmap result directly into
st.mapped. It was probably switched there so that we do not have to
allocate/free the struct when the map operation fails (e.g., because we
don't have the loose object). Before that commit, the struct was passed
in from the caller, so there was no allocation at all.

You can see the leak in the test suite by building with:

  make SANITIZE=leak NO_MMAP=1 CC=clang

and running t1060. We need NO_MMAP so that the mmap() is backed by an
actual malloc(), which allows LSan to detect it. And the leak seems not
to be detected when compiling with gcc, probably due to some internal
compiler decisions about how the stack memory is written.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object-file.c

@@ -2150,7 +2150,7 @@ int odb_source_loose_read_object_stream(struct odb_read_stream **out,
 	return 0;
 error:
 	git_inflate_end(&st->z);
-	munmap(st->mapped, st->mapsize);
+	munmap(mapped, mapsize);
 	free(st);
 	return -1;
 }