cli_check_mydoom_log: Avoid unaligned access.

fmap_need_off_once() may return an unaligned pointer. This in return
leads to an unaligned access during the load of the uint32_t variables
loading to failures on architectures not supporting unaligned access.

This was reported to the Debian BTS as #1073128.

[bigeasy: Commit message, reworked the patch a bit].

Link: https://bugs.debian.org/1073128
Patch-Name: cli_check_mydoom_log-Avoid-unaligned-access.patch
Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

Author: sebastianas

libclamav/special.c

@@ -48,7 +48,8 @@
 
 int cli_check_mydoom_log(cli_ctx *ctx)
 {
-    const uint32_t *record;
+    const uint32_t record[16];
+    const uint32_t *ptr;
     uint32_t check, key;
     fmap_t *map         = ctx->fmap;
     unsigned int blocks = map->len / (8 * 4);
@@ -59,14 +60,25 @@ int cli_check_mydoom_log(cli_ctx *ctx)
     if (blocks > 5)
         blocks = 5;
 
-    record = fmap_need_off_once(map, 0, 8 * 4 * blocks);
-    if (!record)
+    /*
+     * The following pointer might not be properly aligned. There there is
+     * memcmp() + memcpy() workaround to avoid performing an unaligned access
+     * while reading the uint32_t.
+     */
+    ptr = fmap_need_off_once(map, 0, 8 * 4 * blocks);
+    if (!ptr)
         return CL_CLEAN;
+
     while (blocks) { /* This wasn't probably intended but that's what the current code does anyway */
-        if (record[--blocks] == 0xffffffff)
+        const uint32_t marker_ff = 0xffffffff;
+
+        blocks--;
+        if (!memcmp(ptr + blocks, &marker_ff, sizeof(uint32_t)))
             return CL_CLEAN;
     }
 
+    memcpy(record, ptr, sizeof(record));
+
     key   = ~be32_to_host(record[0]);
     check = (be32_to_host(record[1]) ^ key) +
             (be32_to_host(record[2]) ^ key) +