strings.h: Ensure ast_str_buffer(…) returns a 0 terminated string.

If a dynamic string is created with an initial length of 0,
`ast_str_buffer(…)` will return an invalid pointer.

This was a secondary discovery when fixing #65.

(cherry picked from commit 164001b05c)

include/asterisk/strings.h

@@ -753,8 +753,11 @@ char * attribute_pure ast_str_buffer(const struct ast_str *buf),
 	 * being returned; eventually, it should become truly const
 	 * and only be modified via accessor functions
 	 */
+	if (__builtin_expect(buf->__AST_STR_LEN > 0, 1)) {
 		return (char *) buf->__AST_STR_STR;
 	}
+	return "";
+}
 )
 
 /*!