libstdc++: check length in string append [PR103534]

libstdc++: check length in string append [PR103534]

In the testcase for 103534 we get a warning about append leading to memcpy
of a very large number of bytes overflowing the buffer.  This turns out to
be because we weren't calling _M_check_length for string append.  Rather
than do that directly, let's go through the public pointer append that calls
it.

	PR c++/103534

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (append (basic_string)): Call pointer
	append instead of _M_append directly.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Wstringop-overflow-8.C: New test.