Harden default cast options, fix unsafe Python case

Change-Id: I0ec8aa206d1bef3fce3f8fe7239c255bffd3f660

cpp/src/arrow/compute/cast.h

@@ -33,6 +33,8 @@ class FunctionContext;
 class UnaryKernel;
 
 struct CastOptions {
+  CastOptions() : allow_int_overflow(false) {}
+
   bool allow_int_overflow;
 };
 

python/pyarrow/array.pxi

@@ -131,8 +131,8 @@ cdef class Array:
             CCastOptions options
             shared_ptr[CArray] result
 
-        if safe:
-            options.allow_int_overflow = 0
+        if not safe:
+            options.allow_int_overflow = 1
 
         with nogil:
             check_status(Cast(_context(), self.ap[0], target_type.sp_type,