link_alt_odb_entry: fix read over array bounds reported by valgrind

pfxlen can be longer than the path in objdir when relative_base
contains the path to gits object directory.  Here we are interested
in checking if ent->base[] (the part that corresponds to .git/objects)
is the same string as objdir, and the code NUL-terminated ent->base[]
to

	LEADING PATH\0XX/XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX\0

in preparation for these "duplicate check" step (before we return
from the function, the first NUL is turned into '/' so that we can
fill XX when probing for loose objects).  All we need to do is to
compare the string with the path to our object directory.

Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -298,7 +298,7 @@ static int link_alt_odb_entry(const char * entry, int len, const char * relative
 			return -1;
 		}
 	}
-	if (!memcmp(ent->base, objdir, pfxlen)) {
+	if (!strcmp(ent->base, objdir)) {
 		free(ent);
 		return -1;
 	}