r600/sfn: make sure we return a non-negative number of registers

If a shader doesn't use any register and only ssa values we might
end up with zero minimum registers, and because a unsigned is
returned that goes wrong.

Fixes: 565816dfa15214abbeef9a9d94e44f30507ca4d7
    r600/sfn: Set minimum required registers based on array allocation

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/8008

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20516>

src/gallium/drivers/r600/sfn/sfn_shader.cpp

@@ -520,7 +520,8 @@ Shader::allocate_local_registers(const exec_list *registers)
 {
    if (value_factory().allocate_registers(registers))
       m_indirect_files |= 1 << TGSI_FILE_TEMPORARY;
-   m_required_registers = value_factory().next_register_index() - 1;
+   m_required_registers = value_factory().next_register_index() ?
+                             value_factory().next_register_index() - 1 : 0;
 }
 
 bool