pstore/ram: Avoid NULL deref in ftrace merging failure path

Given corruption in the ftrace records, it might be possible to allocate
tmp_prz without assigning prz to it, but still marking it as needing to
be freed, which would cause at least a NULL dereference.

smatch warnings:
fs/pstore/ram.c:340 ramoops_pstore_read() error: we previously assumed 'prz' could be null (see line 255)

https://lists.01.org/pipermail/kbuild-all/2018-December/055528.htmlReported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 2fbea82b ("pstore: Merge per-CPU ftrace records into one")
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/ram.c

@@ -291,6 +291,7 @@ static ssize_t ramoops_pstore_read(struct pstore_record *record)
 					  GFP_KERNEL);
 			if (!tmp_prz)
 				return -ENOMEM;
+			prz = tmp_prz;
 			free_prz = true;
 
 			while (cxt->ftrace_read_cnt < cxt->max_ftrace_cnt) {
@@ -309,7 +310,6 @@ static ssize_t ramoops_pstore_read(struct pstore_record *record)
 					goto out;
 			}
 			record->id = 0;
-			prz = tmp_prz;
 		}
 	}