arg_reader: Add check if range becomes discontiguous

A basic_scan_file_buffer, when it has buffering, it can become
discontiguous while moving forward when executing
ranges::next(range.begin(), ranges::distance(crange.begin(), it)).
For that reason, we add a fallback in case this happened.

Author: spyridon97

src/scn/impl.h

@@ -5913,8 +5913,12 @@ struct default_arg_reader {
             }
             auto crange = get_as_contiguous(range);
             SCN_TRY(it, impl(rd, crange, value));
-            return ranges::next(range.begin(),
-                                ranges::distance(crange.begin(), it));
+            auto result = ranges::next(range.begin(),
+                                       ranges::distance(crange.begin(), it));
+            if (SCN_UNLIKELY(!is_segment_contiguous(range))) {
+                return impl(rd, range, value);
+            }
+            return result;
         }
         else {
             SCN_EXPECT(false);
@@ -6233,8 +6237,12 @@ struct arg_reader {
 
             auto crange = get_as_contiguous(range);
             SCN_TRY(it, impl(rd, crange, value));
-            return ranges::next(range.begin(),
-                                ranges::distance(crange.begin(), it));
+            auto result = ranges::next(range.begin(),
+                                       ranges::distance(crange.begin(), it));
+            if (SCN_UNLIKELY(!is_segment_contiguous(range))) {
+                return impl(rd, range, value);
+            }
+            return result;
         }
         else {
             SCN_EXPECT(false);