libselinux: limit has buffer size

The `struct selabel_digest` member `hashbuf_size` is used to compute
hashes via `Sha1Update()`, which takes uint32_t as length parameter
type.  Use that same type for `hashbuf_size` to avoid potential value
truncations, as the overflow check in `digest_add_specfile()` on
`hashbuf_size` is based on it.

    label_support.c: In function ‘digest_gen_hash’:
    label_support.c:125:53: warning: conversion from ‘size_t’ {aka ‘long unsigned int’} to ‘uint32_t’ {aka ‘unsigned int’} may change value [-Wconversion]
      125 |         Sha1Update(&context, digest->hashbuf, digest->hashbuf_size);
          |                                               ~~~~~~^~~~~~~~~~~~~~

Acked-by: James Carter <jwcart2@gmail.com>
Signed-off-by: Christian Göttsche <cgzones@googlemail.com>

libselinux/src/label_internal.h

@@ -57,7 +57,7 @@ int selabel_service_init(struct selabel_handle *rec,
 struct selabel_digest {
 	unsigned char *digest;	/* SHA1 digest of specfiles */
 	unsigned char *hashbuf;	/* buffer to hold specfiles */
-	size_t hashbuf_size;	/* buffer size */
+	uint32_t hashbuf_size;	/* buffer size */
 	size_t specfile_cnt;	/* how many specfiles processed */
 	char **specfile_list;	/* and their names */
 };