protobuf_c_message_unpack(): initialize tmp.length_prefix_len

this should silence Coverity #1153648, which complains because
tmp.length_prefix_len is uninitialized for certain wire types when
copied on line 2486:

    scanned_member_slabs[which_slab][in_slab_index++] = tmp;
protobuf_c_message_unpack(): reflow some long lines for readability
GenerateServiceDescriptor(): free mi_array, Coverity #1153647
GenerateEnumDescriptor(): free name_index, Coverity #1153646
GenerateEnumDescriptor(): free value_index, Coverity #1153645
GenerateMessageDescriptor(): free field_indices, Coverity #1153644
ChangeLog: coverity fixes