tree-wide: check memstream buffer after closing the handle

When closing the FILE handle attached to a memstream, it may attempt to
do a realloc() that may fail during OOM situations, in which case we are
left with the buffer pointer pointing to NULL and buffer size > 0. For
example:

```
    #include <errno.h>
    #include <stdio.h>
    #include <stdlib.h>

    void *realloc(void *ptr, size_t size) {
        return NULL;
    }

    int main(int argc, char *argv[])
    {
        FILE *f;
        char *buf;
        size_t sz = 0;

        f = open_memstream(&buf, &sz);
        if (!f)
            return -ENOMEM;

        fputs("Hello", f);

        fflush(f);
        printf("buf: 0x%lx, sz: %lu, errno: %d\n",
                    (unsigned long) buf, sz, errno);
        fclose(f);
        printf("buf: 0x%lx, sz: %lu, errno: %d\n",
                    (unsigned long) buf, sz, errno);

        return 0;
    }
```

```
$ gcc -o main main.c
$ ./main
buf: 0x74d4a0, sz: 5, errno: 0
buf: 0x0, sz: 5, errno: 0
```

This might do unexpected things if the underlying code expects a valid
pointer to the memstream buffer after closing the handle.

Found by Nallocfuzz.

2 files changed, 8 insertions, 0 deletions

diff --git a/src/oom/oomd-manager.c b/src/oom/oomd-manager.c
index 08a29ec77b..3f050cdbb2 100644
--- a/src/oom/oomd-manager.c
+++ b/src/oom/oomd-manager.c
@@ -847,6 +847,9 @@ int manager_get_dump_string(Manager *m, char **ret) {
 
         f = safe_fclose(f);
 
+        if (!dump)
+                return -ENOMEM;
+
         *ret = TAKE_PTR(dump);
         return 0;
 }
diff --git a/src/oom/oomd-util.c b/src/oom/oomd-util.c
index 49c10b5e16..6309d2c473 100644
--- a/src/oom/oomd-util.c
+++ b/src/oom/oomd-util.c
@@ -299,6 +299,11 @@ static int dump_kill_candidates(OomdCGroupContext **sorted, int n, int dump_unti
         if (r < 0)
                 return r;
 
+        f = safe_fclose(f);
+
+        if (!dump)
+                return -ENOMEM;
+
         return log_dump(LOG_INFO, dump);
 }