obmcutil: Silence kill error message by quoting test variable

Similar to the last fix, avoid an always-true test result with proper
variable quoting.

Change-Id: I90d79efcc92e61bf5d4989c5675de8dc1928aaff
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/obmcutil b/obmcutil
index 703c129..24bce75 100755
--- a/obmcutil
+++ b/obmcutil
@@ -100,7 +100,7 @@
         "within timeout period (${timeout}s)"
     fi
 
-    if [ -n $verbose_child ]; then
+    if [ -n "$verbose_child" ]; then
         kill $verbose_child
     fi
 }