[Fix #1340] Handle call with no receiver correctly

For blocks with an unused argument which call a method with no explicit
receiver, the Style/SymbolProc would attempt to check 'receiver.type'
and crash because receiver is nil.
Merge pull request #1341 from smangelsdorf/symbol-proc-crash

[Fix #1340] Handle call with no receiver correctly in Style/SymbolProc cop
Cut 0.26.1