MINOR: ist: Add a function to retrieve the ist pointer

There is already the istlen() function to get the ist length. Now, it is
possible to call istptr() to get the ist pointer.
diff --git a/include/common/ist.h b/include/common/ist.h
index 68c83ff..2b5178f 100644
--- a/include/common/ist.h
+++ b/include/common/ist.h
@@ -211,6 +211,12 @@
 	return ist.ptr;
 }
 
+/* returns the pointer of the string */
+static inline char *istptr(const struct ist ist)
+{
+	return ist.ptr;
+}
+
 /* returns the length of the string */
 static inline size_t istlen(const struct ist ist)
 {