Flag pattern definitions with empty RHS as errors

Crashed in pickler before.

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3076,6 +3076,8 @@ object Parsers {
           if (isBackquoted(id)) vdef.pushAttachment(Backquoted, ())
           finalizeDef(vdef, mods, start)
         case _ =>
+          if rhs.isEmpty then
+            syntaxError(ExpectedTokenButFound(EQUALS, in.token), Span(in.lastOffset))
           PatDef(mods, lhs, tpt, rhs)
       }
     }

tests/neg/deferred-patdef.scala

@@ -0,0 +1,4 @@
+object M {
+  val (x, y): (Int, Int)  // error: `=` expected
+  val 1: Int              // error: `=` expected
+}