fsck: use strbuf to generate alternate directories

When fsck-ing alternates, we make a copy of the alternate
directory in a fixed PATH_MAX buffer. We memcpy directly,
without any check whether we are overflowing the buffer.
This is OK if PATH_MAX is a true representation of the
maximum path on the system, because any path here will have
already been vetted by the alternates subsystem. But that is
not true on every system, so we should be more careful.

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

builtin/fsck.c

@@ -683,11 +683,12 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 
 		prepare_alt_odb();
 		for (alt = alt_odb_list; alt; alt = alt->next) {
-			char namebuf[PATH_MAX];
-			int namelen = alt->name - alt->base;
-			memcpy(namebuf, alt->base, namelen);
-			namebuf[namelen - 1] = 0;
-			fsck_object_dir(namebuf);
+			/* directory name, minus trailing slash */
+			size_t namelen = alt->name - alt->base - 1;
+			struct strbuf name = STRBUF_INIT;
+			strbuf_add(&name, alt->base, namelen);
+			fsck_object_dir(name.buf);
+			strbuf_release(&name);
 		}
 	}