shared/install: avoid overwriting 'r' counter with a partial result

We want to store either the first error or the total number of changes in 'r'.
Instead, we were overwriting this with the return value from
install_info_traverse().

LGTM complained later in the loop that:
> Comparison is always true because r >= 0.

src/shared/install.c

@@ -1896,10 +1896,10 @@ static int install_context_apply(
                 if (q < 0)
                         return q;
 
-                r = install_info_traverse(scope, c, paths, i, flags, NULL);
-                if (r < 0) {
+                q = install_info_traverse(scope, c, paths, i, flags, NULL);
+                if (q < 0) {
                         unit_file_changes_add(changes, n_changes, r, i->name, NULL);
-                        return r;
+                        return q;
                 }
 
                 /* We can attempt to process a masked unit when a different unit