memory-related bugs

A suitably-instrumented run of "make installcheck-world" under valgrind turned
up a handful of memory-related bugs:

* memcpy()/strncpy() between overlapping regions
uniqueentry() and dispell_lexize() both deduplicate an array by iteratively
copying elements downward to occlude the duplicates.  Before finding a first
duplicate, these functions call memcpy() with identical arguments.  Similarly,
resolve_polymorphic_tupdesc() calls TupleDescInitEntry() with an attributeName
pointing directly into the TupleDesc's name bytes, causing the latter to call
strncpy() with identical arguments.  The attached mem1v1-memcpy-overlap.patch
fixes these sites by checking for equal pointers before the affected call.  For
TupleDescInitEntry(), I considered instead having resolve_polymorphic_tupdesc()
pstrdup() the value.

* read past the end of a Form_pg_type in examine_attribute()
examine_attribute() copies a Form_pg_type naively.  Since the nullable columns
at the end of the structure are not present in memory, the memcpy() reads eight
bytes past the end of the source allocation.  mem2v1-analyze-overread.patch
updates this code to match how we address the same issue for Form_pg_attribute.

* off-by-one error in shift_jis_20042euc_jis_2004()
This function grabs two bytes at a time, even when only one byte remains; this
makes it read past the end of the input.  mem3v1-sjis-offbyone.patch changes it
to not do this and to report an error when the input ends in a byte that would
start a two-byte sequence.

Thanks,
nm