Fix signed-unsigned comparison.

Envoy compilation fails due to -Wsign-compare, because char is signed in Envoy.
Even though Balsa is compiled with this warning internally, it compiles
internally because char is unsigned.  Change `c` to `size_t` to make sure the
comparison to `std::numeric_limits<size_t>::max() - length_x_10` is always
between two unsigned expressions.

PiperOrigin-RevId: 445979285

quiche/common/balsa/balsa_frame.cc

diff --git a/quiche/common/balsa/balsa_frame.cc b/quiche/common/balsa/balsa_frame.cc
index 5f9d9d2..3762772 100644
--- a/quiche/common/balsa/balsa_frame.cc
+++ b/quiche/common/balsa/balsa_frame.cc
@@ -487,7 +487,7 @@
     }
     const size_t kMaxDiv10 = std::numeric_limits<size_t>::max() / 10;
     size_t length_x_10 = *length * 10;
-    const char c = *value_begin - '0';
+    const size_t c = *value_begin - '0';
     if (*length > kMaxDiv10 ||
         (std::numeric_limits<size_t>::max() - length_x_10) < c) {
       QUICHE_DVLOG(1) << "content-length overflow";