Use unsigned type as index for array.

Use uint8_t instead of char as an index for std::array<> in
buildInvalidHeaderKeyCharLookupTable() and IsInvalidHeaderKeyChar().  Envoy has
signed char, so this change is necessary to ensure that the index is within
bounds.

PiperOrigin-RevId: 446460704

quiche/common/balsa/balsa_frame.cc

diff --git a/quiche/common/balsa/balsa_frame.cc b/quiche/common/balsa/balsa_frame.cc
index 3762772..b148a42 100644
--- a/quiche/common/balsa/balsa_frame.cc
+++ b/quiche/common/balsa/balsa_frame.cc
@@ -37,13 +37,13 @@
 std::array<bool, 256> buildInvalidHeaderKeyCharLookupTable() {
   std::array<bool, 256> invalidCharTable;
   invalidCharTable.fill(false);
-  for (char c : BalsaFrame::kInvalidHeaderKeyCharList) {
+  for (uint8_t c : BalsaFrame::kInvalidHeaderKeyCharList) {
     invalidCharTable[c] = true;
   }
   return invalidCharTable;
 }
 
-inline bool IsInvalidHeaderKeyChar(char c) {
+inline bool IsInvalidHeaderKeyChar(uint8_t c) {
   static const std::array<bool, 256> invalidHeaderKeyCharTable =
       buildInvalidHeaderKeyCharLookupTable();