[PATCH] gdb: initialize the data_head variable to eliminate compilation warnings

[Enze Li <enze.li@hotmail.com>]

On a machine with gcc 12, I get this warning:

  CXX    nat/linux-btrace.o
In function ‘btrace_error linux_read_bts(btrace_data_bts*, btrace_target_info*, btrace_read_type)’,
    inlined from ‘btrace_error linux_read_btrace(btrace_data*, btrace_target_info*, btrace_read_type)’ at ../gdb/nat/linux-btrace.c:935:29:
../gdb/nat/linux-btrace.c:865:21: warning: ‘data_head’ may be used uninitialized [-Wmaybe-uninitialized]
  865 |   pevent->last_head = data_head;
      |   ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~
../gdb/nat/linux-btrace.c: In function ‘btrace_error linux_read_btrace(btrace_data*, btrace_target_info*, btrace_read_type)’:
../gdb/nat/linux-btrace.c:792:9: note: ‘data_head’ was declared here
  792 |   __u64 data_head, data_tail;
      |         ^~~~~~~~~

Fix this by initializing the 'data_head' variable.

Tested by rebuilding on x86_64 openSUSE Tumbleweed with gcc 12.
---
 gdb/nat/linux-btrace.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gdb/nat/linux-btrace.c b/gdb/nat/linux-btrace.c
index b0d6dcd7cf1..c31fb5ffe43 100644
--- a/gdb/nat/linux-btrace.c
+++ b/gdb/nat/linux-btrace.c
@@ -789,7 +789,7 @@ linux_read_bts (struct btrace_data_bts *btrace,
   struct perf_event_buffer *pevent;
   const uint8_t *begin, *end, *start;
   size_t buffer_size, size;
-  __u64 data_head, data_tail;
+  __u64 data_head = 0, data_tail;
   unsigned int retries = 5;
 
   pevent = &tinfo->variant.bts.bts;
-- 
2.36.1