common: Fix uninitialized reps in test-jsonfds

In practice the loop always sets it, but it triggers static code
checkers:

> src/common/test-jsonfds.c:320:32:
> warning: 'reps' may be used uninitialized in this function [-Wmaybe-uninitialized]

Side issue in https://issues.redhat.com/browse/RHEL-104798

Author: martinpitt

src/common/test-jsonfds.c

@@ -273,7 +273,7 @@ test_print_string_memory_safety (TestFixture   *fixture,
         {
           guint residual = seq;
           gint length = 0;
-          int reps;
+          int reps = 0;
 
           fixture->expected_end = stpcpy (fixture->expected_buffer, ", \"key\": \"");