[PLAT-8434] Fix improper use of TCHAR_TO_UTF8

[nickdowell]

Goal

Fix a customer-reported use-after-free crash in FAndroidPlatformJNI::ParseFString() when passed a large string, due to improper use of TCHAR_TO_UTF8().

StringConv.h states that results should not be captured in a variable:

/**
 * NOTE: The objects these macros declare have very short lifetimes. They are
 * meant to be used as parameters to functions. You cannot assign a variable
 * to the contents of the converted string as the object will go out of
 * scope and the string released.
 *
 * NOTE: The parameter you pass in MUST be a proper string, as the parameter
 * is typecast to a pointer. If you pass in a char, not char* it will compile
 * and then crash at runtime.
 *
 * Usage:
 *
 *		SomeApi(TCHAR_TO_ANSI(SomeUnicodeString));
 *
 *		const char* SomePointer = TCHAR_TO_ANSI(SomeUnicodeString); <--- Bad!!!
 */

UE's string conversion use different allocation strategies depending on the size of the data, switching at 128 bytes.

Changeset

Uses TCHAR_TO_UTF8() in the recommended way - as a parameter to NewStringUTF()

There is no need for a null check since, given a non-null pointer (which FString::operator*() returns), TCHAR_TO_ANSI() will also return a non-null pointer.

Testing

Was unable to reproduce a crash, even when testing with large strings of over 128 bytes.