glsl/glcpp: Fix NULL directives when followed by a single-line comment

glsl/glcpp: Fix NULL directives when followed by a single-line comment

This is the fix for the following line:

	#  // comment to ignore here

According to the translation-phase rules, the comment should be removed before
the preprocessor looks to interpret the null directive.

So in our implementation we must explicitly look for single-line comments in
the <HASH> start condition as well.

This commit fixes the following Khronos GLES3 CTS tests:

	null_directive_vertex
	null_directive_fragment

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>