[flang][unittests] Fix recent snprintf() changes to use correct buffer lengths

[klausler]

Summary

A recent patch replaced two sprintf() calls with snprintf(), but didn't
use the right length for the remaining space in the buffer.

[klausler]

clang-format

[cseo]

LGTM.

flang/unittests/Decimal/quick-sanity-test.cpp

if (result.str == nullptr) {
     float y{0};
     char *q{const_cast<char *>(result.str)};
     int expo{result.decimalExponent};
     expo -= result.length;
     if (*q == '-' || *q == '+') {
       ++expo;
     }
     if (q >= buffer && q < buffer + sizeof buffer) {
-      std::snprintf(q + result.length, sizeof buffer, "e%d", expo);
+      std::snprintf(q + result.length,
+          buffer + sizeof buffer - (q + result.length), "e%d", expo);
     }
     const char *p{q};
     auto rflags{ConvertDecimalToFloat(&p, &y, RoundNearest)};
     union u u;
     if (!(x == x)) {
       if (y == y || *p != '\0' || (rflags & Invalid)) {
         u.x = y;
         (failed(x) << " (NaN) " << flags << ": -> '" << result.str << "' -> 0x")

flang/unittests/Decimal/thorough-test.cpp

if (result.str == nullptr) {
     char *q{const_cast<char *>(result.str)};
     if ((*q >= '0' && *q <= '9') ||
         ((*q == '-' || *q == '+') && q[1] >= '0' && q[1] <= '9')) {
       int expo{result.decimalExponent};
       expo -= result.length;
       if (*q == '-' || *q == '+') {
         ++expo;
       }
-      std::snprintf(q + result.length, sizeof buffer, "e%d", expo);
+      std::snprintf(q + result.length,
+          buffer + sizeof buffer - (q + result.length), "e%d", expo);
     }
     const char *p{q};
     auto rflags{ConvertDecimalToFloat(&p, &y, RoundNearest)};
     if (!(x == x)) {
       if (y == y || *p != '\0' || (rflags & Invalid)) {
         u.x = y;
         failed(x) << " (NaN) " << flags << ": -> '" << result.str << "' -> 0x";
         failed(x).write_hex(u.u) << " '" << p << "' " << rflags << '\n';