util/cbfstool: Initialize elf_writer pointer to avoid crash

If some error happens in cbfs_payload_make_elf, the code jumps to "out",
and elf_writer_destroy(ew) is called. This may happen before an elf
writer is allocated.
To avoid accessing an uninitialized pointer, initialize ew to NULL;
elf_writer_destroy will perform no action in this case.

Change-Id: I5f1f9c4d37f2bdeaaeeca7a15720c7b4c963d953
Reported-By: Coverity Scan (1361475)
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/16124
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

util/cbfstool/cbfs_image.c

@@ -1052,7 +1052,7 @@ static int cbfs_payload_make_elf(struct buffer *buff, uint32_t arch)
 	Elf64_Ehdr ehdr;
 	Elf64_Shdr shdr;
 	struct cbfs_payload_segment *segs = NULL;
-	struct elf_writer *ew;
+	struct elf_writer *ew = NULL;
 	struct buffer elf_out;
 	int segments = 0;
 	int retval = -1;