Check return value in fmt_addr

Previously, if tor_addr_to_str() returned NULL, we would reuse the
last value returned by fmt_addr().  (This could happen if we were
erroneously asked to format an AF_UNSPEC address.)  Now instead we
return "???".

changes/fmt_addr

@@ -0,0 +1,4 @@
+  o Minor bugfixes:
+    - When unable to format an address as a string, report its value
+      as "???" rather than reusing the last formatted address. Bugfix
+      on 0.2.1.5-alpha.

src/common/address.c

@@ -958,8 +958,10 @@ fmt_addr(const tor_addr_t *addr)
 {
   static char buf[TOR_ADDR_BUF_LEN];
   if (!addr) return "<null>";
-  tor_addr_to_str(buf, addr, sizeof(buf), 0);
-  return buf;
+  if (tor_addr_to_str(buf, addr, sizeof(buf), 0))
+    return buf;
+  else
+    return "???";
 }
 
 /** Convert the string in <b>src</b> to a tor_addr_t <b>addr</b>.  The string