[rubygems/rubygems] Fix Kernel#warn override to handle backtrace location with nil path

It's very unlikely to hit this case, but it is possible, as
Thread::Backtrace::Location#path can return nil if the location is
a cfunc with no previous iseq.  See location_path in vm_backtrace.c
in Ruby.

https://github.com/rubygems/rubygems/commit/511935645a

lib/rubygems/core_ext/kernel_warn.rb

@@ -38,12 +38,13 @@ if RUBY_VERSION >= "2.5"
 
           start += 1
 
-          path = loc.path
+          if path = loc.path
             unless path.start_with?(rubygems_path) or path.start_with?('<internal:')
               # Non-rubygems frames
               uplevel -= 1
             end
           end
+        end
         uplevel = start
       end