Rubocop wants us not to use !=0

Let's revert conditions and use `zero?` predicate.

Addressing rubocop comments like:
C: Use **.nonzero? instead of ** != 0.

Author: isimluk

lib/openscap/source.rb

@@ -37,7 +37,7 @@ def type
 
     def validate!
       e = FFI::MemoryPointer.new(:char, 4096)
-      if 0 != OpenSCAP.oscap_source_validate(@raw, XmlReporterCallback, e)
+      unless OpenSCAP.oscap_source_validate(@raw, XmlReporterCallback, e).zero?
         OpenSCAP.raise! e.read_string
       end
     end

lib/openscap/xccdf/session.rb

@@ -32,7 +32,7 @@ def load(opts = {})
           OpenSCAP.xccdf_session_set_datastream_id(@s, o[:datastream_id])
           OpenSCAP.xccdf_session_set_component_id(@s, o[:component_id])
         end
-        OpenSCAP.raise! if OpenSCAP.xccdf_session_load(@s) != 0
+        OpenSCAP.raise! unless OpenSCAP.xccdf_session_load(@s).zero?
         OpenSCAP.raise! unless OpenSCAP.xccdf_session_load_check_engine_plugins(@s) == 0
       end
 
@@ -44,7 +44,7 @@ def profile=(p)
       end
 
       def evaluate
-        OpenSCAP.raise! if OpenSCAP.xccdf_session_evaluate(@s) != 0
+        OpenSCAP.raise! unless OpenSCAP.xccdf_session_evaluate(@s).zero?
       end
 
       def remediate