initialize default_config to supress compiler warning

[fcelda]

Amalgamated build of bcd yields a new warning on Fedora 43 with GCC 15.2.1 and source fortification enabled.

Steps to reproduce:

./configure
make amalgamated
gcc -I include -O2 -Wall -Werror -Wp,-U_FORTIFY_SOURCE,-D_FORTIFY_SOURCE=3 -o /tmp/bcd-amalgamated.o -c src/bcd-amalgamated.c

Observed output:

In function ‘bcd_config_assign’,
    inlined from ‘bcd_init’ at src/bcd-amalgamated.c:2608:9:
src/bcd-amalgamated.c:3076:23: error: ‘*(const struct bcd_config *)((char *)&default_config + offsetof(bcd_config_latest_version_t, version)).version’ may be used uninitialized [-Werror=maybe-uninitialized]
 3076 |         switch (bcd_cf->version) {
      |                 ~~~~~~^~~~~~~~~
src/bcd-amalgamated.c: In function ‘bcd_init’:
src/bcd-amalgamated.c:2600:45: note: ‘default_config’ declared here
 2600 |                 bcd_config_latest_version_t default_config;
      |                                             ^~~~~~~~~~~~~~
cc1: all warnings being treated as errors

This change adds zero initialization of the default_config variable in bcd_init to suppress the warning.

I don't think this is any real issue but the change is harmless and eliminates the warning.