Fix accidental use of = instead of ==

Fix for commit 630f9a43ce.  It used = instead of ==.  The result
would be an incorrect error message.

Author: Jacob Brazeal <jacob.brazeal@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/CA%2BCOZaC-JMbhQ4O0Q8V1Bxa0R%2BNex_RN9D6UyuLPiEx_CK4Heg%40mail.gmail.com

src/backend/commands/indexcmds.c

@@ -2472,7 +2472,7 @@ GetOperatorFromCompareType(Oid opclass, Oid rhstype, CompareType cmptype,
 		if (*strat == InvalidStrategy)
 			ereport(ERROR,
 					errcode(ERRCODE_UNDEFINED_OBJECT),
-					cmptype = COMPARE_EQ ? errmsg("could not identify an equality operator for type %s", format_type_be(opcintype)) :
+					cmptype == COMPARE_EQ ? errmsg("could not identify an equality operator for type %s", format_type_be(opcintype)) :
 					cmptype == COMPARE_OVERLAP ? errmsg("could not identify an overlaps operator for type %s", format_type_be(opcintype)) :
 					cmptype == COMPARE_CONTAINED_BY ? errmsg("could not identify a contained-by operator for type %s", format_type_be(opcintype)) : 0,
 					errdetail("Could not translate compare type %d for operator family \"%s\", input type %s, access method \"%s\".",