Binary fuzzer fails for required fields

[vishwacs111]

The binary format parser uses ParsePartialFromString while parsing the message:

libprotobuf-mutator/src/binary_format.cc

Line 28 in a304ec4

if (!output->ParsePartialFromString(data)) {

However the LoadProtoInput calls mutator fix which expects an initialized message (i.e. with all the required fields set).

I think the binary format parser should use ParseFromString instead of ParsePartialFromString.