qdist: return "(empty)" instead of NULL when printing an empty dist

Printf'ing a NULL string is undefined behaviour. Avoid it.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
Message-Id: <1469459025-23606-4-git-send-email-cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/test-qdist.c

@@ -360,10 +360,16 @@ static void test_none(void)
     g_assert(isnan(qdist_xmax(&dist)));
 
     pr = qdist_pr_plain(&dist, 0);
-    g_assert(pr == NULL);
+    g_assert_cmpstr(pr, ==, "(empty)");
+    g_free(pr);
 
     pr = qdist_pr_plain(&dist, 2);
-    g_assert(pr == NULL);
+    g_assert_cmpstr(pr, ==, "(empty)");
+    g_free(pr);
+
+    pr = qdist_pr(&dist, 0, QDIST_PR_BORDER);
+    g_assert_cmpstr(pr, ==, "(empty)");
+    g_free(pr);
 
     qdist_destroy(&dist);
 }

util/qdist.c

@@ -14,6 +14,8 @@
 #define NAN (0.0 / 0.0)
 #endif
 
+#define QDIST_EMPTY_STR "(empty)"
+
 void qdist_init(struct qdist *dist)
 {
     dist->entries = g_new(struct qdist_entry, 1);
@@ -233,7 +235,7 @@ char *qdist_pr_plain(const struct qdist *dist, size_t n)
     char *ret;
 
     if (dist->n == 0) {
-        return NULL;
+        return g_strdup(QDIST_EMPTY_STR);
     }
     qdist_bin__internal(&binned, dist, n);
     ret = qdist_pr_internal(&binned);
@@ -308,7 +310,7 @@ char *qdist_pr(const struct qdist *dist, size_t n_bins, uint32_t opt)
     GString *s;
 
     if (dist->n == 0) {
-        return NULL;
+        return g_strdup(QDIST_EMPTY_STR);
     }
 
     s = g_string_new("");