Check for undefined source.symbol

Fixes #37014

Author: RyanCavanaugh

src/compiler/checker.ts

@@ -16239,6 +16239,7 @@ namespace ts {
                 if (unmatchedProperty.valueDeclaration
                     && isNamedDeclaration(unmatchedProperty.valueDeclaration)
                     && isPrivateIdentifier(unmatchedProperty.valueDeclaration.name)
+                    && source.symbol
                     && source.symbol.flags & SymbolFlags.Class) {
                     const privateIdentifierDescription = unmatchedProperty.valueDeclaration.name.escapedText;
                     const symbolTableKey = getSymbolNameForPrivateIdentifier(source.symbol, privateIdentifierDescription);

tests/cases/compiler/privateFieldAssignabilityFromUnknown.ts

@@ -0,0 +1,5 @@
+export class Class {
+  #field: any
+}
+
+const task: Class = {} as unknown;