snprintf off by one.

Summary:
According to `man snprintf`

    Upon successful return, these functions return the number of
    characters printed (not including the trailing '\0' used to end
    output to strings).

Or am I misunderstanding something about what's going on here?

public

Reviewed By: zlern2k, dcolascione

Differential Revision: D2805759

fb-gh-sync-id: f20908f80cdfff9677fe38c7c1cf733f0f5b1c5e

ReactAndroid/src/main/jni/first-party/jni/fbjni/Exceptions.cpp

@@ -193,7 +193,7 @@ void setNewJavaException(jclass exceptionClass, const char* fmt, ARGS... args) {
   JNIEnv* env = internal::getEnv();
 
   try {
-    char *msg = (char*) alloca(msgSize);
+    char *msg = (char*) alloca(msgSize + 1);
     snprintf(msg, kMaxExceptionMessageBufferSize, fmt, args...);
     env->ThrowNew(exceptionClass, msg);
   } catch (...) {

ReactAndroid/src/main/jni/first-party/jni/fbjni/Exceptions.h

@@ -114,7 +114,7 @@ template<typename... Args>
   assertIfExceptionsNotInitialized();
   int msgSize = snprintf(nullptr, 0, fmt, args...);
 
-  char *msg = (char*) alloca(msgSize);
+  char *msg = (char*) alloca(msgSize + 1);
   snprintf(msg, kMaxExceptionMessageBufferSize, fmt, args...);
   throwNewJavaException(throwableName, msg);
 }