Fix some sparse warnings

Sparse issues some "Using plain integer as NULL pointer" warnings.
Each warning relates to the use of an '{0}' initialiser expression
in the declaration of an 'struct object_info'. The first field of
this structure has pointer type. Thus, in order to suppress these
warnings, we replace the initialiser expression with '{NULL}'.
Signed-off-by: NRamsay Jones <ramsay@ramsay1.demon.co.uk>
Acked-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

sha1_file.c

@@ -2477,7 +2477,7 @@ int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi)
 int sha1_object_info(const unsigned char *sha1, unsigned long *sizep)
 {
 	enum object_type type;
-	struct object_info oi = {0};
+	struct object_info oi = {NULL};
 
 	oi.typep = &type;
 	oi.sizep = sizep;

streaming.c

@@ -135,7 +135,7 @@ struct git_istream *open_istream(const unsigned char *sha1,
 				 struct stream_filter *filter)
 {
 	struct git_istream *st;
-	struct object_info oi = {0};
+	struct object_info oi = {NULL};
 	const unsigned char *real = lookup_replace_object(sha1);
 	enum input_source src = istream_source(real, type, &oi);