[Libc] avoid spurious uninitialized warnings from the compiler

Clang 13 emits warnings related to uninitialized variables:

variable 'r' is used uninitialized whenever 'if'
condition is false [-Wsometimes-uninitialized]

This is due to the fact that the assert(0) macro, when using Frama-C's libc,
is expanded to __FC_assert. Since it is not known by the compiler, it cannot
consider it as "noreturn" when the condition is zero. Note that adding
'__attribute__ ((__noreturn__))' will not work: Frama-C will assume it
never returns, even when the condition is true.
Initializing the 'r' variables with a default value will not change the
actual behavior, and will avoid the warnings.