Check the alignment in posix_memalign() earlier

Make sure all conditions to do a proper memory allocation are satisfied
before a zero size memory allocation is performed.

Update #4390.

cpukit/libcsupport/src/posix_memalign.c

@@ -37,10 +37,6 @@ int posix_memalign(
 
   *memptr = NULL;
 
-  if ( size == 0 ) {
-    return 0;
-  }
-
   if ( alignment < sizeof( void * ) ) {
     return EINVAL;
   }
@@ -49,6 +45,10 @@ int posix_memalign(
     return EINVAL;
   }
 
+  if ( size == 0 ) {
+    return 0;
+  }
+
   *memptr = rtems_heap_allocate_aligned_with_boundary( size, alignment, 0 );
 
   if ( *memptr == NULL ) {