malloc: simplify nasm_malloc code, add nasm_strcatn()

Simplify the nasm_malloc() code by moving the pointer check into a
common subroutine.

We can now issue a filename error even for failures like malloc().

Add support for the gcc sentinel attribute (verify that a list ends
with NULL).

Add a handful of safe_alloc attributes.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

include/compiler.h

@@ -264,7 +264,7 @@ size_t strnlen(const char *s, size_t maxlen);
 #ifdef HAVE_FUNC_ATTRIBUTE_MALLOC
 # define safe_alloc never_null __attribute__((malloc))
 #else
-# define safe_alloc
+# define safe_alloc never_null
 #endif
 
 #ifdef HAVE_FUNC_ATTRIBUTE_ALLOC_SIZE
@@ -277,6 +277,12 @@ size_t strnlen(const char *s, size_t maxlen);
 # define safe_realloc(s) never_null
 #endif
 
+#ifdef HAVE_FUNC_ATTRIBUTE_SENTINEL
+# define end_with_null __attribute__((sentinel))
+#else
+# define end_with_null
+#endif
+
 /*
  * How to tell the compiler that a function doesn't return
  */