buffer: rely on `GITERR_OOM` set by `git_buf_try_grow`

The function `git_buf_try_grow` consistently calls `giterr_set_oom`
whenever growing the buffer fails due to insufficient memory being
available. So in fact, we do not have to do this ourselves when a call
to any buffer-growing function has failed due to an OOM situation. But
we still do so in two functions, which this patch cleans up.

src/pack.c

@@ -324,7 +324,6 @@ static int pack_index_open(struct git_pack_file *p)
 	git_buf_put(&idx_name, p->pack_name, name_len - strlen(".pack"));
 	git_buf_puts(&idx_name, ".idx");
 	if (git_buf_oom(&idx_name)) {
-		giterr_set_oom();
 		return -1;
 	}
 

src/transports/winhttp.c

@@ -429,7 +429,6 @@ static int winhttp_stream_connect(winhttp_stream *s)
 			git_buf_printf(&processed_url, ":%s", t->proxy_connection_data.port);
 
 		if (git_buf_oom(&processed_url)) {
-			giterr_set_oom();
 			error = -1;
 			goto on_error;
 		}