TypeError when using String-Field with value parameter

[lukaspanni]

Checklist

• I have included the [relevant portions of the] grammar used that caused the bug
• I have filled out the environment section

Environment

Platform

• Windows
• Mac
• Linux
• Other (please specify)

Python Version

• Python 2.7
• Python 3.4
• Python 3.5
• Python 3.6
• Python 3.7
• Python 3.8
• Other (please specify)

Describe the bug

When using a String-Field with predefined value and min/max this error randomly occurs:
TypeError: 'bytes' object cannot be interpreted as an integer

gramfuzz tries to use the value of the String-Field as length which fails because the value is a string.

To Reproduce

Grammar:

class TRef(Ref):
    cat = "test_string_error"

class TDef(Def):
    cat = "test_string_error"

Def("error_test", TRef("field"), cat="transport_header")

TDef("field", String("TEST", min=0, max=8))

Execute

    fuzzer = gramfuzz.GramFuzzer()
    gramfuzz.rand.seed(2)
    fuzzer.load_grammar("grammar.py")
    # will trigger the error, if 2 instead of 3 is used no error occurs
    data = fuzzer.gen(3, cat="test_string_error")  

Expected Behavior

gramfuzz generates multiple strings with different lengths as stated in the grammar.

[d0c-s4vage]

I should be able to get to these this weekend. Thank you for reporting the issue!