glsl_to_nir: Zero nir_constant in constant_copy for valgrind & nir_serialize

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/compiler/glsl/glsl_to_nir.cpp

@@ -219,7 +219,7 @@ constant_copy(ir_constant *ir, void *mem_ctx)
    if (ir == NULL)
       return NULL;
 
-   nir_constant *ret = ralloc(mem_ctx, nir_constant);
+   nir_constant *ret = rzalloc(mem_ctx, nir_constant);
 
    const unsigned rows = ir->type->vector_elements;
    const unsigned cols = ir->type->matrix_columns;