[partition] Avoid uninitialized variable

- if the partition size is invalid, then warn about it but do
  not print the (uninitialized) size of the partition.
- shuffle code to continue earlier, allowing the "good path"
  code to be out-dented.

src/modules/partition/core/PartitionLayout.cpp

@@ -172,12 +172,16 @@ PartitionLayout::execute( Device* dev,
     // Let's check if we have enough space for each partSize
     for ( const auto& part : qAsConst( m_partLayout ) )
     {
-        qint64 size;
-        // Calculate partition size
-
-        if ( part.partSize.isValid() )
+        if ( !part.partSize.isValid() )
         {
-            // We need to ignore the percent-defined
+            cWarning() << "Partition" << part.partMountPoint << "size is invalid, skipping...";
+            continue;
+        }
+
+        // Calculate partition size: Rely on "possibly uninitialized use"
+        // warnings to ensure that all the cases are covered below.
+        qint64 size;
+        // We need to ignore the percent-defined until later
         if ( part.partSize.unit() != CalamaresUtils::Partition::SizeUnit::Percent )
         {
             size = part.partSize.toSectors( totalSize, dev->logicalSize() );
@@ -193,12 +197,6 @@ PartitionLayout::execute( Device* dev,
                 size = 0;
             }
         }
-        }
-        else
-        {
-            cWarning() << "Partition" << part.partMountPoint << "size (" << size << "sectors) is invalid, skipping...";
-            continue;
-        }
 
         partSizeMap.insert( &part, size );
         availableSize -= size;