loader: check get_image_size() return value

since a negative value means it errored.

hw/core/loader.c:149:9: warning: Loss of sign in implicit conversion
    if (size > max_sz) {
        ^~~~
hw/core/loader.c:171:9: warning: Loss of sign in implicit conversion
    if (size > memory_region_size(mr)) {
        ^~~~

Reported-by: Clang Static Analyzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
