Fix backwards test in operator_precedence_warning logic.

Warnings about unary minus might have been wrong.  It's a bit
surprising that nobody noticed yet ... probably the precedence-warning
feature hasn't really been used much in the field.

Rikard Falkeborn

Discussion: https://postgr.es/m/CADRDgG6fzA8A2oeygUw4=o7ywo4kvz26NxCSgpq22nMD73Bx4Q@mail.gmail.com

src/backend/parser/parse_expr.c

@@ -3236,7 +3236,7 @@ operator_precedence_group(Node *node, const char **nodename)
 				*nodename = strVal(linitial(aexpr->name));
 				/* Ignore if op was always higher priority than IS-tests */
 				if (strcmp(*nodename, "+") == 0 ||
-					strcmp(*nodename, "-"))
+					strcmp(*nodename, "-") == 0)
 					group = 0;
 				else
 					group = PREC_GROUP_PREFIX_OP;