Squash shift warnings in ssh2_bpp_check_unimplemented.

I did a horrible thing with a list macro which builds up a 256-bit
bitmap of known SSH-2 message types at compile time, by means of
evaluating a conditional expression per known message type and per
bitmap word which boils down to (in pseudocode)

  (shift count in range ? 1 << shift count : 0)

I think this is perfectly valid C. If the shift count is out of range,
then the use of the << operator in the true branch of the ?: would
have undefined behaviour if it were executed - but that's OK, because
in that situation, the safe false branch is executed instead.

But when the whole thing is a compile-time evaluated constant
expression, the compiler can prove statically that the << in the true
branch is an out-of-range shift, and at least some compilers will warn
about it verbosely. The same compiler *could* also prove statically
that that branch isn't taken, and use that to suppress the warning -
but at least clang does not.

The solution is the same one I used in shift_right_by_one_word and
shift_left_by_one_word in mpint.c: inside the true branch, nest a
second conditional expression which coerces the shift count to always
be in range, by setting it to 0 if it's not. This doesn't affect the
output, because the only cases in which the output of the true branch
is altered by this transformation are the ones in which the true
branch wasn't taken anyway.

So this change should make no difference to the output of this macro
construction, but it suppresses about 350 pointless warnings from
clang.

ssh/common.c

@@ -787,7 +787,9 @@ void ssh2_bpp_queue_disconnect(BinaryPacketProtocol *bpp,
 }
 
 #define BITMAP_UNIVERSAL(y, name, value)                        \
-    | (value >= y && value < y+32 ? 1UL << (value-y) : 0)
+    | (value >= y && value < y+32                               \
+       ? 1UL << (value >= y && value < y+32 ? (value-y) : 0)    \
+       : 0)
 #define BITMAP_CONDITIONAL(y, name, value, ctx) \
     BITMAP_UNIVERSAL(y, name, value)
 #define SSH2_BITMAP_WORD(y) \