i965/vec4: Fix hypothetical use of uninitialized data in attribute_map[].

i965/vec4: Fix hypothetical use of uninitialized data in attribute_map[].

Fixes issue identified by Klocwork analysis:

    'attribute_map' array elements might be used uninitialized in this
    function (vec4_visitor::lower_attributes_to_hw_regs).

The attribute_map array contains the mapping from shader input
attributes to the hardware registers they are stored in.
vec4_vs_visitor::setup_attributes() only populates elements of this
array which, according to core Mesa, are actually used by the shader.
Therefore, when vec4_visitor::lower_attributes_to_hw_regs() accesses
the array to lower a register access in the shader, it should in
principle only access elements of attribute_map that contain valid
data.  However, if a bug ever caused the driver back-end to access an
input that was not flagged as used by core Mesa, then
lower_attributes_to_hw_regs() would access uninitialized memory, which
could cause illegal instructions to get generated, resulting in a
possible GPU hang.

This patch makes the situation more robust by using memset() to
pre-initialize the attribute_map array to zero, so that if such a bug
ever occurred, lower_attributes_to_hw_regs() would generate a (mostly)
harmless access to r0.  In addition, it adds assertions to
lower_attributes_to_hw_regs() so that if we do have such a bug, we're
likely to discover it quickly.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>