uboot-phytec

tools: zynqmpimage: Check return values from file functions

Check all return values from file functions.
In case of negative return exit immediately.
Also change fsize return value which can't be negative.

Reported-by: Coverity (CID: 23276, 23304, 169357)
Signed-off-by: Michal Simek <michal.simek@xilinx.com>