[RFA] Fix leaks when pruning inferiors.

[Philippe Waroquiers <philippe.waroquiers@skynet.be>]

Valgrind detects various inferior related leaks, such as:
  ==31877== 5,530 (56 direct, 5,474 indirect) bytes in 1 blocks are definitely lost in loss record 7,131 of 7,355
  ==31877==    at 0x4C2E18C: calloc (vg_replace_malloc.c:760)
  ==31877==    by 0x23E580: xcalloc (alloc.c:100)
  ==31877==    by 0x4794A9: xcnewvec<void*> (poison.h:158)
  ==31877==    by 0x4794A9: registry_alloc_data(registry_data_registry*, registry_fields*) (registry.c:51)
  ==31877==    by 0x3A537C: inferior_alloc_data (inferior.c:43)
  ==31877==    by 0x3A537C: inferior::inferior(int) (inferior.c:92)
  ==31877==    by 0x3A5426: add_inferior_silent(int) (inferior.c:98)
  ==31877==    by 0x3A5530: add_inferior(int) (inferior.c:122)
  ...

Origin of the leaks is in prune_inferiors: prune_inferiors is first removing
the inferior to prune from the inferior list, then calls delete_inferior.
But delete_inferior will only really destroy the inferior when it finds
it into the inferior list.
As delete_inferior is removing the inferior to delete from the inferior list,
ensure prune_inferiors only calls delete_inferior, without touching the
inferior list.

gdb/ChangeLog
YYYY-MM-DD  Philippe Waroquiers  <philippe.waroquiers@skynet.be>
	* inferior.c (prune_inferiors):  Only call delete_inferior,
	Do not modify the inferior list.
---
 gdb/inferior.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/gdb/inferior.c b/gdb/inferior.c
index 84e4d24e73..55c8409cd9 100644
--- a/gdb/inferior.c
+++ b/gdb/inferior.c
@@ -370,24 +370,22 @@ have_live_inferiors (void)
 void
 prune_inferiors (void)
 {
-  struct inferior *ss, **ss_link;
+  struct inferior *ss, *ss_next;
 
   ss = inferior_list;
-  ss_link = &inferior_list;
   while (ss)
     {
       if (!ss->deletable ()
 	  || !ss->removable
 	  || ss->pid != 0)
 	{
-	  ss_link = &ss->next;
-	  ss = *ss_link;
+	  ss = ss->next;
 	  continue;
 	}
 
-      *ss_link = ss->next;
+      ss_next = ss->next;
       delete_inferior (ss);
-      ss = *ss_link;
+      ss = ss_next;
     }
 }
 
-- 
2.20.1