monitor: Catch printing to non-existent monitor

The monitor_vprintf() function now touches the 'mon' pointer
before calling monitor_puts(), this causes block migration
to segfault as its functions call monitor_printf() with a
NULL 'mon'.

To fix the problem this commit moves the 'mon' NULL check
from monitor_puts() to monitor_vprintf().

This can potentially hide bugs, but for some reason this has
been the behavior for a long time.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

monitor.c

diff --git a/monitor.c b/monitor.c
index b518cc4..ebd0282 100644
--- a/monitor.c
+++ b/monitor.c
@@ -177,9 +177,6 @@
 {
     char c;
 
-    if (!mon)
-        return;
-
     for(;;) {
         c = *str++;
         if (c == '\0')
@@ -195,6 +192,9 @@
 
 void monitor_vprintf(Monitor *mon, const char *fmt, va_list ap)
 {
+    if (!mon)
+        return;
+
     if (mon->mc && !mon->mc->print_enabled) {
         qemu_error_new(QERR_UNDEFINED_ERROR);
     } else {