Fix rubocop Style/IfUnlessModifier

Addressing:
lib/openscap/source.rb:40:7: C: Style/IfUnlessModifier: Favor modifier unless usage when having a single-line body. Another good alternative is the usage of control flow &&/||.
      unless OpenSCAP.oscap_source_validate(@raw, XmlReporterCallback, e).zero?
      ^^^^^^

Author: isimluk

lib/openscap/source.rb

@@ -37,9 +37,7 @@ def type
 
     def validate!
       e = FFI::MemoryPointer.new(:char, 4096)
-      unless OpenSCAP.oscap_source_validate(@raw, XmlReporterCallback, e).zero?
-        OpenSCAP.raise! e.read_string
-      end
+      OpenSCAP.raise!(e.read_string) unless OpenSCAP.oscap_source_validate(@raw, XmlReporterCallback, e).zero?
     end
 
     def save(filepath = nil)