tools: mkimage: Print human readable error when -d is not specified

tools: mkimage: Print human readable error when -d is not specified

When asking mkimage to create a new image file and option -d is not
specified then mkimage show human unfriendly error message:

  mkimage: Can't open (null): Bad address

Without debugger it is hard to debug what is the issue. Function open() is
being called with file name set to NULL. So add a check for this and if it
happens then show human readable message that option -d was not specified.

Signed-off-by: Pali Rohár <pali@kernel.org>