lib: cbprintf: do not blindly skip tags

When CONFIG_LOG_USE_TAGGED_ARGUMENTS is enabled, and
CONFIG_CBPRINTF_COMPLETE is also enabled, we should not be
blindly skipping tags when processing the tagged package
for output.  The issue is that if there is a "%%" in
the format string, the specifier is considered invalid but
the code blindly skips ahead in the argument list as if
it is a valid specifier (think "%s"), which resulting in
the next valid specifier using incorrect argument in
the list. So fix it by skipping ahead if and only if
the specifier is not invalid.

Fixes #68271

Signed-off-by: Daniel Leung <daniel.leung@intel.com>