exec: move warning of null argv to be next to the relevant code

Problem: The warning is currently printed where it is detected that the
arg count is zero but the action is only taken place later in the flow
even though the warning is written as if the action is taken place in
the time of print

This could be problematic since there could be a failure between the
print and the code that takes action which would deem this warning
misleading

Solution: Move the warning print after the action of adding an empty
string as the first argument is successful

Signed-off-by: Nir Lichtman <nir@lichtman.org>
Link: https://lore.kernel.org/r/ZyYUgiPc8A8i_3FH@nirs-laptop

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Signed-off-by: Kees Cook <kees@kernel.org>