[rubygems/rubygems] Fix Kernel#warn override to handle backtrace loca…

…tion 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.

rubygems/rubygems@511935645a

lib/rubygems/core_ext/kernel_warn.rb

@@ -38,10 +38,11 @@ class << self
 
           start += 1
 
-          path = loc.path
-          unless path.start_with?(rubygems_path) or path.start_with?('<internal:')
-            # Non-rubygems frames
-            uplevel -= 1
+          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