amf.h, amf.c (amf_str): amf_str was transformed from a macro into a function for better sanity checks.
amf.c (amf_string_read): Avoided allocating an empty buffer for zero-length strings.
amf.c (amf_string_new): Avoided copying null strings into newly-created AMF strings.
amf.c (amf_data_dump): Fixed a bug where zero-length strings would cause fprintf to segfault flvdump in Linux.