GDB: Fix out of bounds accesses with limited-length values

Fix accesses to limited-length values in `contents_copy_raw' and
`contents_copy_raw_bitwise' so that they observe the limit of the
original allocation.

Reported by Simon Marchi as a heap-buffer-overflow AddressSanitizer
issue triggered with gdb.ada/limited-length.exp.

Approved-By: Simon Marchi <simon.marchi@efficios.com>