mesa: ensure that variable is initialized

This variable controls whether we link using the glsl code path or the
spirv path. It's set when we validate that all shaders are glsl or
spirv, but if there are no shaders attached to the program it will
remain unset, resulting in undefined behavior. We want to go down the
glsl path in that case, so initialize to false.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105820
Fixes: 16f6634e7f
       ("mesa/program: Link SPIR-V shaders using the SPIR-V code-path")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Tested-by: Mark Janes <mark.a.janes@intel.com>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>

src/mesa/program/ir_to_mesa.cpp

@@ -3126,7 +3126,7 @@ void
 _mesa_glsl_link_shader(struct gl_context *ctx, struct gl_shader_program *prog)
 {
    unsigned int i;
-   bool spirv;
+   bool spirv = false;
 
    _mesa_clear_shader_program_data(ctx, prog);