GL encoder null pointer check before write

GL encoder null pointer check before write

Previously when encoding some functions, if the user gives us a null
pointer, the encoder will write a header with some data length while not
actually writing any data. This patch fix it by writing 0 into the data
length field for all those functions.

The changed functions include:
glBufferSubData
glCompressedSubTexImage2D
glTexSubImage3DOES
glCompressedTexImage3DOES
glCompressedTexSubImage3DOES

Other functions do not directly write user's pointer, so they don't need
to be updated.

Since this is auto-generated code, there would be another patch in
emu-master-dev to update the code generator.

Change-Id: I05fca078dfcf01e20fa757f1050820b21ca080b7