patch 9.0.0845: shell command with just space gives strange error

Problem:    Shell command with just space gives strange error.
Solution:   Skip white space at start of the argument. (Christian Brabandt,
            Shane-XB-Qian, closes #11515, closes #11495)
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 7bb7aa0..3cf07e4 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -899,11 +899,13 @@
     }
 
     /*
-     * Try to find an embedded bang, like in :!<cmd> ! [args]
-     * (:!! is indicated by the 'forceit' variable)
+     * Try to find an embedded bang, like in ":!<cmd> ! [args]"
+     * ":!!" is indicated by the 'forceit' variable.
      */
     ins_prevcmd = forceit;
-    trailarg = arg;
+
+    // Skip leading white space to avoid a strange error with some shells.
+    trailarg = skipwhite(arg);
     do
     {
 	len = (int)STRLEN(trailarg) + 1;