Fix test crashes I just introduced.

src/runtime.cc

Index: src/runtime.cc
diff --git a/src/runtime.cc b/src/runtime.cc
index dcec8384d9c0370836a9c26d50d4315d99d5cad8..a4dbd79ec2c6320062aa346e6d3cdae01c72fbc6 100644
--- a/src/runtime.cc
+++ b/src/runtime.cc
@@ -2444,7 +2444,7 @@ static Object* ConvertCase(Arguments args,
   // in the buffer
   Access<StringInputBuffer> buffer(&string_input_buffer);
   buffer->Reset(s);
-  unibrow::uchar chars[unibrow::kMaxMappingSize];
+  unibrow::uchar chars[Converter::kMaxWidth];
   int i = 0;
   // We can assume that the string is not empty
   uc32 current = buffer->GetNext();