Silence compiler warnings on clang 21
Clang 21 shows some new compiler warnings, for example:
warning: variable 'dstsize' is uninitialized when passed as a const pointer argument here [-Wuninitialized-const-pointer]
The fix is to initialize the variables when they are defined. This is
similar to, for example, the existing situation in gistKeyIsEQ().
Discussion: https://www.postgresql.org/message-id/flat/
6604ad6e-5934-43ac-8590-
15113d6ae4b1%40eisentraut.org