Fix nit in logblocks_alloc_from_logregion() with already allocated blocks

When some blocks were already allocated, we would fail to
use all of the log region, that is, the newly added assertion

	if (n > 0) AZ(logreg->free_n);

would fail

This left some blocks of the logregion unused, but was insignificant
otherwise.

src/fellow_log.c

@@ -2099,10 +2099,10 @@ logblocks_alloc_from_logregion(struct fellow_log_region *logreg,
 	AN(logreg->free_off);
 	assert(region_contains(logreg->region, logreg->free_off));
 
-	if (logreg->free_n < n)
-		n = logreg->free_n;
-
-	for (i = 0; i < n; i++, arr++) {
+	for (i = 0;
+	     n > 0 && logreg->free_n > 0;
+	     n--, arr++) {
+		i++;
 		if (arr->off != 0)
 			continue;
 		AN(logreg->free_n);
@@ -2112,6 +2112,9 @@ logblocks_alloc_from_logregion(struct fellow_log_region *logreg,
 		logreg->free_n--;
 	}
 
+	if (n > 0)
+		AZ(logreg->free_n);
+
 	if (logreg->free_n == 0) {
 		assert(logreg->free_off ==
 		    logreg->region->off + (off_t)logreg->region->size);