Temporary patches around LSET and negative layers

Probably this should be replaced with a less error-prone
approach.  Right now the LSET -> BASE_SET system is risky
because it is converting a signed enum (PCB_LAYER_ID) to
a size_t in all the underlying operations.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/18738

common/lset.cpp

@@ -45,7 +45,10 @@ LSET::LSET( std::initializer_list<PCB_LAYER_ID> aList ) :
     LSET()
 {
     for( PCB_LAYER_ID layer : aList )
-        set( layer );
+    {
+        if( layer > 0 )
+            set( layer );
+    }
 }
 
 
@@ -53,14 +56,20 @@ LSET::LSET( const LSEQ& aSeq ) :
     LSET()
 {
     for( PCB_LAYER_ID layer : aSeq )
-        set( layer );
+    {
+        if( layer > 0 )
+            set( layer );
+    }
 }
 
 
 LSET::LSET( const LAYER_RANGE& aRange )
 {
     for( PCB_LAYER_ID layer : aRange )
-        set( layer );
+    {
+        if( layer > 0 )
+            set( layer );
+    }
 }
 
 
@@ -966,4 +975,4 @@ LSET::non_copper_layers_iterator LSET::non_copper_layers_end() const
 }
 
 
-#endif
+#endif

include/lset.h

@@ -59,6 +59,10 @@ public:
      */
     bool Contains( PCB_LAYER_ID aLayer )
     {
+        // At the moment, LSET cannot store negative layers, but PCB_LAYER_ID can contain them
+        if( aLayer < 0 )
+           return false;
+
         try
         {
             return test( aLayer );