st/mesa: fix texture memory allocation bug

st/mesa: fix texture memory allocation bug

The following example caused an incorrect GL_OUT_OF_MEMORY error to be
raised in glTexSubImage2D:

   glTexImage2D(level=0, width=32, height=32, pixels=NULL);
   glTexImage2D(level=0, width=64, height=64, pixels=NULL);
   glTexSubImage2D(level=0, pixels!=NULL);

The second glTexImage2D() call needs to cause the first image to be
deallocated then reallocated at the new size.  This was not happening
because we were testing for pixels==NULL too early.