Fix unused return value warnings which manifest as errors. (#1396)

This should be the correct fix for the build issues that came up on
recent ECH changes.

Background: Unless you install Clang and take steps to make it the
default compiler, then  on many Linux distributions (esp Ubuntu) the
JNI code will get built with gcc.

gcc does not consider casting to void is sufficient to avoid a warning
about unused returns from a function which has the warn_unused_result
attribute.

This change assigns the result to a variable with the CONSCRYPT_UNUSED
attribute which works correctly on Clang, gcc and MSVC.

As for why it's OK in these cases:

The asprintf methods should probably *not* ignore the value but
they're calling FatalError with the result so it's game over anyway.
This method could use re-writing but not this PR.

The read and write on fdsEmergency in native_crypto is part of an
"interesting" mechanism to prevent blocking forever when using the
ConscryptFileDescriptorSocket implementation, which ought to go away
but works as-is.

Author: prbprbprb

common/src/jni/main/cpp/conscrypt/jniutil.cc

@@ -159,14 +159,15 @@ void jniRegisterNativeMethods(JNIEnv* env, const char* className, const JNINativ
     ScopedLocalRef<jclass> c(env, env->FindClass(className));
     if (c.get() == nullptr) {
         char* msg;
-        (void)asprintf(&msg, "Native registration unable to find class '%s'; aborting...",
-                       className);
+        CONSCRYPT_UNUSED int n = asprintf(
+                &msg, "Native registration unable to find class '%s'; aborting...", className);
         env->FatalError(msg);
     }
 
     if (env->RegisterNatives(c.get(), gMethods, numMethods) < 0) {
         char* msg;
-        (void)asprintf(&msg, "RegisterNatives failed for '%s'; aborting...", className);
+        CONSCRYPT_UNUSED int n =
+                asprintf(&msg, "RegisterNatives failed for '%s'; aborting...", className);
         env->FatalError(msg);
     }
 }

common/src/jni/main/cpp/conscrypt/native_crypto.cc

@@ -7835,7 +7835,7 @@ static int sslSelect(JNIEnv* env, int type, jobject fdObject, AppData* appData,
         if (fds[1].revents & POLLIN) {
             char token;
             do {
-                (void)read(appData->fdsEmergency[0], &token, 1);
+                CONSCRYPT_UNUSED int n = read(appData->fdsEmergency[0], &token, 1);
             } while (errno == EINTR);
         }
     }
@@ -7866,7 +7866,7 @@ static void sslNotify(AppData* appData) {
     char token = '*';
     do {
         errno = 0;
-        (void)write(appData->fdsEmergency[1], &token, 1);
+        CONSCRYPT_UNUSED int n = write(appData->fdsEmergency[1], &token, 1);
     } while (errno == EINTR);
     errno = errnoBackup;
 #endif