fix ply file header parsing

[Slonegg]

I have an assertion with debug PCL when parsing ply file saved by CloudCompare. I think this pull request should fix it.

ply
format binary_little_endian 1.0
comment Author: CloudCompare (TELECOM PARISTECH/EDF R&D)
obj_info Generated by CloudCompare!
element vertex 15070
property float x
property float y
property float z
property uchar red
property uchar green
property uchar blue
property float nx
property float ny
property float nz
element face 29996
property list uchar int vertex_indices
end_header

[taketwo]

This assert served to guard accesses to st[2] in following lines. Would be nice if the patch preserved this behavior as well.

[Slonegg]

@taketwo you right, I wasn't paying enough attention. I think it's reasonable to have if there, seems that obj_info can be followed by any string.