Don't pass NULL to memcpy in gdb

Don't pass NULL to memcpy in gdb

I compiled gdb with -fsanitize=undefined and ran the test suite.

A couple of reports came from passing NULL to memcpy, e.g.:

[...]btrace-common.cc:176:13: runtime error: null pointer passed as argument 2, which is declared to never be null

While it would be better to fix this in the standard, in the meantime
it seems easy to avoid this error.

gdb/ChangeLog
2020-03-31  Tom Tromey  <tromey@adacore.com>

	* dwarf2/abbrev.c (abbrev_table::read): Conditionally call
	memcpy.

gdbsupport/ChangeLog
2020-03-31  Tom Tromey  <tromey@adacore.com>

	* btrace-common.cc (btrace_data_append): Conditionally call
	memcpy.