ART: Add zero-padding to allocation request for String.

Also make the signed-to-unsigned conversion of utf16_length
in String::Alloc() explicit and make the overflow check take
into account the oveflow in the multiplication by
sizeof(uint16_t), i.e. being passed a negative utf16_length.

Bug: 23528461
Change-Id: Ia5844ea79f8f12e897f52fa27b0984358e3dea0b
1 file changed