RFR(S): 8145015: jni_GetStringCritical asserts for empty strings

[Volker Simonis]

Hi,

can somebody please review and sponsor the following fix:

http://cr.openjdk.java.net/~simonis/webrevs/2015/8145015/
https://bugs.openjdk.java.net/browse/JDK-8145015

The problem is that change "8141132: JEP 254: Compact Strings" removed
the handling for empty strings from jni_GetStringCritical(). As a
consequence, the functions will now assert with:

assert(is_within_bounds(which)) failed: index 0 out of bounds 0

if called with a string of length zero (because it tries to access the
zeroth element of a zero-length array).

The fix is trivial, just use 's_value->base(T_CHAR)' instead of
's_value->char_at_addr(0)'.

While doing this fix I also noticed that jni_GetStringCritical()
doesn't handle out-of-memory situations when creating a new character
array (before this wasn't necessary because it always returned the
original character array, but now with compressed strings it may have
to allocate a new array). So I've also added the new check by using
NEW_C_HEAP_ARRAY_RETURN_NULL() instead of NEW_C_HEAP_ARRAY().

Notice that I also allocate one extra character for zero termination.
While it seems that this is not strictly necessary, it is now the same
code like in jni_GetStringChars(). And it again simplifies the
handling of zero-length strings. Without the extra character it would
be hard to distinguish out-of-memory errors from the allocation of
zero bytes, because malloc() is free to return NULL if called with an
allocation size of zero.

Thank you and best regards,
Volker