Fix asserts in fast-path locking code

Commit c4d5cb71d2 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

src/backend/storage/lmgr/lock.c

@@ -218,8 +218,8 @@ int			FastPathLockGroupsPerBackend = 0;
  * of fast-path lock slots.
  */
 #define FAST_PATH_SLOT(group, index) \
-	(AssertMacro(((group) >= 0) && ((group) < FastPathLockGroupsPerBackend)), \
-	 AssertMacro(((index) >= 0) && ((index) < FP_LOCK_SLOTS_PER_GROUP)), \
+	(AssertMacro((uint32) (group) < FastPathLockGroupsPerBackend), \
+	 AssertMacro((uint32) (index) < FP_LOCK_SLOTS_PER_GROUP), \
 	 ((group) * FP_LOCK_SLOTS_PER_GROUP + (index)))
 
 /*
@@ -227,10 +227,10 @@ int			FastPathLockGroupsPerBackend = 0;
  * the FAST_PATH_SLOT macro, split it into group and index (in the group).
  */
 #define FAST_PATH_GROUP(index)	\
-	(AssertMacro(((index) >= 0) && ((index) < FP_LOCK_SLOTS_PER_BACKEND)), \
+	(AssertMacro((uint32) (index) < FP_LOCK_SLOTS_PER_BACKEND), \
 	 ((index) / FP_LOCK_SLOTS_PER_GROUP))
 #define FAST_PATH_INDEX(index)	\
-	(AssertMacro(((index) >= 0) && ((index) < FP_LOCK_SLOTS_PER_BACKEND)), \
+	(AssertMacro((uint32) (index) < FP_LOCK_SLOTS_PER_BACKEND), \
 	 ((index) % FP_LOCK_SLOTS_PER_GROUP))
 
 /* Macros for manipulating proc->fpLockBits */

src/backend/storage/lmgr/proc.c

@@ -322,7 +322,7 @@ InitProcGlobal(void)
 	}
 
 	/* Should have consumed exactly the expected amount of fast-path memory. */
-	Assert(fpPtr = fpEndPtr);
+	Assert(fpPtr == fpEndPtr);
 
 	/*
 	 * Save pointers to the blocks of PGPROC structures reserved for auxiliary