config: fix "maybe-uninitialized" error when compiling with CFLAGS=-Og

Using GCC 12.2.0 with the following build steps:

    CFLAGS=-Og meson bld
    ninja -C bld

...produces the compiler error:

    ../config.c:2000:18: error: ‘sym_equal’ may be used uninitialized
    [-Werror=maybe-uninitialized]

This commit fixes that by using BUG() to assert that all possible
values are accounted for in the offending switch statement.

config.c

@@ -2008,6 +2008,9 @@ resolve_key_binding_collisions(struct config *conf, const char *section_name,
                 sym_equal = (binding1->m.button == binding2->m.button &&
                              binding1->m.count == binding2->m.count);
                 break;
+
+            default:
+                BUG("unhandled key binding type");
             }
 
             if (!mods_equal || !sym_equal)