glsl: more accurately handle swizzle in 64bit varying split with no left value

as implied in the surrounding code, left_components can be 0 here, in which case creating a left swizzle is unnecessary (and triggers an assert)

this moves a failing assert farther down the stack to a more useful location when trying to pack e.g., struct[3] { dvec3; float; }

ref spec@arb_gpu_shader_fp64@execution@inout@vs-out-fs-in-s1-s2@3-dvec2-float