[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

     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)) {

flang/unittests/Decimal/thorough-test.cpp

       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)) {