Avoid undefined behavior with msvc compiler (src/include/port/pg_bitutils.h)

Hi,

The pg_leftmost_one_pos32 function with msvc compiler,
relies on Assert to guarantee the correct result.

But msvc documentation [1] says clearly that
undefined behavior can occur.

Fix by checking the result of the function to avoid
a possible undefined behavior.

patch attached.

best regards,
Ranier Vilela

[1]
https://learn.microsoft.com/en-us/cpp/intrinsics/bitscanreverse-bitscanreverse64?view=msvc-170