What does “Pourvu que ça tienne !” mean in this context?
I think it could possibly have two meanings.
The most probable meaning (espérons que) is exactly what pat27 said, which could also be translated as "Let's hope it (the car) holds up/stays in one piece" or in order to capture more fully the sense of "pourvu que" and Milou's fear of the consequences if the car in fact fails to hold up, perhaps the following:
For the sake of our own survival, let's hope that it holds up.
The other meaning (à condition que), although probably a little far-fetched, but perhaps slightly indicated by the ellipses following Tintin's statement is that Milou/Snowy is actually finishing Tintin's statement with
AS LONG AS (instead of "Let's hope") it (the car) holds up/stays in one piece,
which would give the meaning of "He's going to bomb us again if/only if (provided that) the car holds up" (slightly optimistic, I suppose) or the more pessimistic "He'll continue dropping bombs as long as/(so long as) the car holds up."