clover: Don't return CL_INVALID_VALUE if there is no header.
clover: Don't return CL_INVALID_VALUE if there is no header.

Reviewed-by: Francisco Jerez <5906ff32bdd9fd8db196f6ba5ad3afd6f9257ea5@riseup.net>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler