policy/fees: remove a floating-point division by zero

Reported-by: practicalswift <practicalswift@users.noreply.github.com>
Signed-off-by: Antoine Poinsot <darosior@protonmail.com>

test/sanitizer_suppressions/ubsan

@@ -1,6 +1,5 @@
 # -fsanitize=undefined suppressions
 # =================================
-float-divide-by-zero:policy/fees.cpp
 float-divide-by-zero:validation.cpp
 float-divide-by-zero:wallet/wallet.cpp