Fix asserts in fast-path locking code

Fix asserts in fast-path locking code

Commit c4d5cb71d229 introduced a couple asserts in the fast-path locking
code, upsetting Coverity.

The assert in InitProcGlobal() is clearly wrong, as it assigns instead
of checking the value. This is harmless, but doesn't check anything.

The asserts in FAST_PATH_ macros are written as if for signed values,
but the macros are only called for unsigned ones. That makes the check
for (val >= 0) useless. Checks written as ((uint32) x < max) work for
both signed and unsigned values. Negative values should wrap to values
greater than INT32_MAX.

Per Coverity, report by Tom Lane.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/2891628.1727019959@sss.pgh.pa.us