Fix checking return value of fread()

The latest code cannot execute a *.mrb including debug information. (compiled with "-g" option)

* test.rb
```
undefined_method
```


* current
```
$ bin/mrbc test.rb
$ bin/mruby -b test.mrb
trace:
NoMethodError: undefined method 'undefined_method' for main
```
```
$ bin/mrbc -g test.rb
$ bin/mruby -b test.mrb
failed to load mrb file: test.mrb
```

* fixed
```
$ bin/mrbc test.rb
$ bin/mruby -b test.mrb
trace:
NoMethodError: undefined method 'undefined_method' for main
```
```
$ bin/mrbc -g test.rb
$ bin/mruby -b test.mrb
trace:
        [0] test.rb:1
test.rb:1: undefined method 'undefined_method' for main (NoMethodError)
```