Fix zenity memory issues

[actondev]

If the user cancels the operation (ie hitting escape), stdOut is not
null, but en empty string

The issue can easily be observed with asan.

gcc ./test/test_opendialog.c ./src/nfd_common.c ./src/nfd_zenity.c -I./src/include -g -fsanitize=address 
./a.out

When ran in current develop:

==120899==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x71f6921e004f at pc 0x5e096915280a bp 0x7ffe5e0a4cb0 sp 0x7ffe5e0a4ca0
WRITE of size 1 at 0x71f6921e004f thread T0
    #0 0x5e0969152809 in NFD_OpenDialog src/nfd_zenity.c:194
    #1 0x5e096914b660 in main test/test_opendialog.c:13

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