detect failed allocations from posix_memalign

- `posix_memalign` doesn't touch its pointer-to-pointer parameter when it
  fails, so `AlignedAlloc` ended up returning stack garbage in that case.

Author: glessard

include/swift/Basic/Malloc.h

@@ -34,11 +34,11 @@ inline void *AlignedAlloc(size_t size, size_t align) {
   if (align < sizeof(void*))
     align = sizeof(void*);
 
-  void *r;
 #if defined(_WIN32)
-  r = _aligned_malloc(size, align);
+  void *r = _aligned_malloc(size, align);
   assert(r && "_aligned_malloc failed");
 #else
+  void *r = nullptr;
   int res = posix_memalign(&r, align, size);
   assert(res == 0 && "posix_memalign failed");
   (void)res; // Silence the unused variable warning.

test/stdlib/UnsafeRawPointer.swift

@@ -229,4 +229,18 @@ UnsafeMutableRawPointerExtraTestSuite.test("moveInitialize:from:") {
   check(Check.RightOverlap)
 }
 
+UnsafeMutableRawPointerExtraTestSuite.test("absurd.allocation.misaligned") {
+  expectCrashLater()
+  let mustFail = UnsafeMutableRawPointer.allocate(byteCount: 1024,
+                                                  alignment: 19)
+  expectUnreachable()
+}
+
+UnsafeMutableRawPointerExtraTestSuite.test("absurd.allocation.gargantuan") {
+  expectCrashLater()
+  let mustFail = UnsafeMutableRawPointer.allocate(byteCount: 400000000000000000,
+                                                  alignment: 0)
+  expectUnreachable()
+}
+
 runAllTests()