fix MANDATORY_FIELDS comparisons (need to exclude -1)

eeschema/class_libentry.cpp

@@ -1099,7 +1099,7 @@ bool LIB_PART::LoadField( LINE_READER& aLineReader, wxString& aErrorMsg )
         return false;
     }
 
-    if( field->GetId() < MANDATORY_FIELDS )
+    if( (unsigned) field->GetId() < MANDATORY_FIELDS )
     {
         LIB_FIELD* fixedField = GetField( field->GetId() );
 

eeschema/lib_field.cpp

@@ -264,7 +264,7 @@ bool LIB_FIELD::Load( LINE_READER& aLineReader, wxString& errorMsg )
     }
 
     // fields in RAM must always have names.
-    if( m_id < MANDATORY_FIELDS )
+    if( (unsigned) m_id < MANDATORY_FIELDS )
     {
         // Fields in RAM must always have names, because we are trying to get
         // less dependent on field ids and more dependent on names.

eeschema/sch_component.cpp

@@ -895,7 +895,7 @@ void SCH_COMPONENT::UpdateFields( bool aResetStyle, bool aResetRef )
 
             if( idx == REFERENCE && !aResetRef )
                 continue;
-            if( idx < MANDATORY_FIELDS )
+            if( (unsigned) idx < MANDATORY_FIELDS )
                 schField = GetField( idx );
             else
                 schField = FindField( field.GetName() );

eeschema/sch_legacy_plugin.cpp

@@ -2747,7 +2747,7 @@ void SCH_LEGACY_PLUGIN_CACHE::loadField( std::unique_ptr< LIB_PART >& aPart,
     }
 
     // Fields in RAM must always have names.
-    if( id < MANDATORY_FIELDS )
+    if( (unsigned) id < MANDATORY_FIELDS )
     {
         // Fields in RAM must always have names, because we are trying to get
         // less dependent on field ids and more dependent on names.