fix warnings: expression result unused [-Wunused-value]

In the assert()s take advantage of the fact that string constants
("string") are effectively of type 'const char []', which when used in
an expression yield a non-NULL pointer.

An assertion that should always fail can thus be formulated as:

assert(!"fail);

An assertion where a text message should be added to the expression can
be written as such:

assert("message" && expression);

Signed-off-by: Giel van Schijndel <me@mortis.eu>