[libc] Fix strtod hex exponent overflow bug

[michaelrj]

Summary

Same issue as was fixed in commit 3d95323, but for hexadecimal floats.

libc/src/__support/str_to_float.h

if (src[index + 1] == '+' || src[index + 1] == '-' ||
       index += result.parsed_len;
 
       // Here we do this operation as int64 to avoid overflow.
       int64_t temp_exponent = static_cast<int64_t>(exponent) +
                               static_cast<int64_t>(add_to_exponent);
 
       // If the result is in the valid range, then we use it. The valid range is
       // also within the int32 range, so this prevents overflow issues.
-      if (temp_exponent < fputil::FPBits<T>::MAX_EXPONENT &&
-          temp_exponent > -fputil::FPBits<T>::MAX_EXPONENT) {
-        exponent = static_cast<int32_t>(temp_exponent);
-      } else if (temp_exponent > fputil::FPBits<T>::MAX_EXPONENT) {
+      if (temp_exponent > fputil::FPBits<T>::MAX_EXPONENT) {
         exponent = fputil::FPBits<T>::MAX_EXPONENT;
-      } else {
+      } else if (temp_exponent < -fputil::FPBits<T>::MAX_EXPONENT) {
         exponent = -fputil::FPBits<T>::MAX_EXPONENT;
+      } else {
+        exponent = static_cast<int32_t>(temp_exponent);
       }
     }
   }
   output.parsed_len = index;
   if (mantissa == 0) { // if we have a 0, then also 0 the exponent.
     output.value.exponent = 0;
     output.value.mantissa = 0;
   } else {

libc/test/src/stdlib/strtod_test.cpp

run_test(
       "000000000000000000000000000000000000000000000000000000000000000000000000"
       "000000000000000000000000000000000000000000000000000000000000000000000000"
       "000000000000000000000000000000000000000000000005238581124701719460000000"
       "000000000017194600000000000000000070046000000000000000000000000000000000"
       "000000000000000000000000000000000000000000000000000000000000000000000000"
       "200000000000000000E608",
       1462, uint64_t(0x7ff0000000000000), ERANGE);
 
+  // Same as above but for hex.
+  run_test("0x0164810157p2047", 17, uint64_t(0x7ff0000000000000), ERANGE);
+
   // This bug was in the handling of very large exponents in the exponent
   // marker. Previously anything greater than 10,000 would be set to 10,000.
   // This caused incorrect behavior if there were more than 10,000 '0's in the
   // input number, and then a correspondingly large exponent. This test case has
   // 24,744 zeroes.
   run_test(
       "0x."
       "000000000000000000000000000000000000000000000000000000000000000000000000"