libstdc++: Remove bogus assertion in std::from_chars [PR105324]

I'm not sure what I was thinking when I added this assertion, maybe it
was supposed to be alignment == 1 (which is what the pmr::string actually
uses). The simplest fix is to just remove the assertion.

The assertion is no longer enabled by default on trunk, but it's still
there for the --enablke-libstdcxx-debug build, and is still wrong. The
fix is needed on the gcc-11 branch.

2022-04-21  Jonathan Wakely  <jwakely@redhat.com>

	PR libstdc++/105324
	* src/c++17/floating_from_chars.cc (buffer_resource::do_allocate):
	Remove assertion.
	* testsuite/20_util/from_chars/pr105324.cc: New test.

--- libstdc++-v3/src/c++17/floating_from_chars.cc
+++ libstdc++-v3/src/c++17/floating_from_chars.cc
@@ -101,7 +101,6 @@ namespace
 	return m_buf + std::__exchange(m_bytes, m_bytes + bytes);
 
       __glibcxx_assert(m_ptr == nullptr);
-      __glibcxx_assert(alignment != 1);
 
       m_ptr = operator new(bytes);
       m_bytes = bytes;
--- libstdc++-v3/testsuite/20_util/from_chars/pr105324.cc
+++ libstdc++-v3/testsuite/20_util/from_chars/pr105324.cc
@@ -0,0 +1,14 @@
+// { dg-do run { target c++17 } }
+
+#include <charconv>
+#include <string>
+
+int main()
+{
+  // PR libstdc++/105324
+  // std::from_chars() assertion at floating_from_chars.cc:78
+  std::string s(512, '1');
+  s[1] = '.';
+  long double d;
+  std::from_chars(s.data(), s.data() + s.size(), d);
+}