fix: release fd slot on failed file open

[beeget]

This pull request improves error handling in the openFile method of SystemIO.java by ensuring that file descriptors are properly closed if a FileNotFoundException occurs during file open operations. This helps prevent resource leaks when file operations fail.

file_path: .asciz  "./file_that_doesnt_exist"


li a7, 1024 # Open
la a0, file_path
li a1, 0 # read-only
ecall # ret a0 = -1 because the file doesn't exist

li a7, 1024 # Open
la a0, file_path
li a1, 1 # write-only, it should create the file
ecall # ret a0 = -1, instead of creating the file and returning the correct fd, since the fd was never released from the first read-only open.