i386: fix regression parsing multiboot initrd modules

The logic for parsing the multiboot initrd modules was messed up in

  commit 950c4e6c94
  Author: Daniel P. Berrangé <berrange@redhat.com>
  Date:   Mon Apr 16 12:17:43 2018 +0100

    opts: don't silently truncate long option values

Causing the length to be undercounter, and the number of modules over
counted. It also passes NULL to get_opt_value() which was not robust
at accepting a NULL value.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20180514171913.17664-2-berrange@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Tested-by: Roman Kagan <rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/qemu-option.c

@@ -75,7 +75,9 @@ const char *get_opt_value(const char *p, char **value)
     size_t capacity = 0, length;
     const char *offset;
 
-    *value = NULL;
+    if (value) {
+        *value = NULL;
+    }
     while (1) {
         offset = qemu_strchrnul(p, ',');
         length = offset - p;