BUG/MINOR: tools: Add OOM check for malloc() in indent_msg()

BUG/MINOR: tools: Add OOM check for malloc() in indent_msg()

This patch adds a missing out-of-memory (OOM) check after
the call to `malloc()` in `indent_msg()`. If memory
allocation fails, the function returns NULL to prevent
undefined behavior.

Co-authored-by: Christian Norbert Menges <christian.norbert.menges@sap.com>
(cherry picked from commit 26776c7b8f963299585300472a2b29ad39e47943)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 35c11b9b05b1adbfc75338a08660460458507e6f)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit ffbcd48e5af6bcfd25a14ab8bb0553e2954ff0e6)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>