Coverity-415952: Remove logically dead code

In aspack decrypt function, there's a check to make sure that backbytes
doesn't exceed 57, because it is used as an index in init_array.
However, it is mathematically impossible.
So this commit removes the check.

libclamav/aspack.c

@@ -286,7 +286,10 @@ static int decrypt(struct ASPK *stream, uint8_t *stuff, uint32_t size, uint8_t *
             if (!build_decrypt_dictionaries(stream)) return 0;
             continue;
         }
-        if ((backbytes = (gen - 256) >> 3) >= 58) return 0; /* checks init_array + stuff */
+        backbytes = (gen - 256) >> 3;
+        // backbytes is < 720. 719 - 256 = 463. 463 >> 3 = 57 (max).
+        // So backbytes cannot overrun the init_array.
+
         backsize = ((gen - 256) & 7) + 2;
         if ((backsize - 2) == 7) {
             uint8_t hlp;