utils: Don't let ssize_t overflow when reading very large files

The size to be allocated is tracked as ssize_t, so if it's larger than
this, doubling it would cause a signed overflow.

Limiting the data we will read into memory to SSIZE_MAX/2 still lets it
occupy 25% of addressable memory (1 GiB on 32-bit or some very large
amount on 64-bit), which should be adequate. In practice we expect this
function to read a few KiB at most.

In practice we're likely to run out of memory before reaching this
point; changing this to SSIZE_MAX / 8, compiling as 32-bit and running
`${builddir}/bwrap --args 0 < /dev/zero` is a convenient way to test
this code path.

Fixes: 422c078e "Check for allocation size overflows"
Signed-off-by: Simon McVittie <smcv@collabora.com>

utils.c

@@ -19,6 +19,7 @@
 #include "config.h"
 
 #include "utils.h"
+#include <limits.h>
 #include <stdint.h>
 #include <sys/syscall.h>
 #include <sys/socket.h>
@@ -599,7 +600,7 @@ load_file_data (int     fd,
     {
       if (data_len == data_read + 1)
         {
-          if (data_len > SIZE_MAX / 2)
+          if (data_len > SSIZE_MAX / 2)
             {
               errno = EFBIG;
               return NULL;