Remove unnecessary null pointer checks

[elfring]

Extra null pointer checks are not needed in functions like the following.

• list_clients
• solid_bg

💭 I am curious if another software developer (besides me) can get into the mood to apply a corresponding update suggestion which could be generated by the software “Coccinelle” (also with the help of a variant from the following script).

@Remove_unnecessary_pointer_checks@
expression x;
@@
-if (\(x != 0 \| x != NULL\))
    free(x);