make macro safer

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@8525 d0543943-73ff-0310-b7d9-9358b9ac24b2

src/include/switch_utils.h

@@ -154,8 +154,8 @@ SWITCH_DECLARE(unsigned char) switch_char_to_rfc2833(char key);
  */
 #define is_dtmf(key)  ((key > 47 && key < 58) || (key > 64 && key < 69) || (key > 96 && key < 101) || key == 35 || key == 42 || key == 87 || key == 119)
 
-#define end_of(_s) *(_s + strlen(_s) - 1)
-#define end_of_p(_s) (_s + strlen(_s) - 1)
+#define end_of(_s) *(*_s == '\0' ? _s : _s + strlen(_s) - 1)
+#define end_of_p(_s) (*_s == '\0' ? _s : _s + strlen(_s) - 1)
 /*!
   \brief Test for the existance of a flag on an arbitary object
   \param obj the object to test