glsl: Add assert to check variable_entry referenced_count after construction.

glsl: Add assert to check variable_entry referenced_count after construction.

A variable_entry after construction should have its referenced_count
member set to 0. However, occassionally this isn't the case and
entry->referenced_count has been observed to be a garbage value. This
leads to crashes of several tests in the Piglit test suite.

This patch adds an assert to check that a variable_entry instance has
its referenced_count member initialized to 0 after construction.