test: ensure nerrors variable is initialized

There is an error path that jumps over the initialization of
nerrors, and the jump target reads the variable contents.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

src/test/test_driver.c

@@ -3275,7 +3275,7 @@ static int testDomainGetDiskErrors(virDomainPtr dom,
     virDomainObjPtr vm = NULL;
     int ret = -1;
     size_t i;
-    size_t nerrors;
+    size_t nerrors = 0;
 
     virCheckFlags(0, -1);