gdb: Check for not allocated/associated values during array slicing

When extracting an array slice we should give up if the array is
not-allocated or not-associated.  For Fortran, at least in gfortran
compiled code, the upper and lower bounds are undefined if the array
is not allocated or not associated, in which case performing checks
against these bounds will result in undefined behaviour.

Better then to throw an error if we try to slice such an array.  This
changes the error message that the user will receive in these
cases (if they got an error message before).  Previously they may have
gotten "slice out of range" now they'll get "array not allocated" or
"array not associated".

gdb/ChangeLog:

	* valops.c (value_slice): Check for not allocated or not
	associated values.

gdb/testsuite/ChangeLog:

	* gdb.fortran/vla-sizeof.exp: Update expected results.