Analyzer: get_char (STRUCT_FILE sf): ensured the type of character strings:

so, we could analyze one of movie formats mp4, we think sure.

---
i: 19: |2avc1|
i: 20 loop starts. struct_fp.fp |1991220328|
get_char starts.
struct_fp.fp |1991220328| struct_fp.index |20|
struct_fp.fp |1991220328|
get_char ends.
i: 20: |avc1|
i: 21 loop starts. struct_fp.fp |1991220328|
get_char starts.
struct_fp.fp |1991220328| struct_fp.index |21|
struct_fp.fp |1991220328|
get_char ends.
i: 21: |vc1|
...
we could find moov.
---