Treat zero requested size in GC_malloc_many same as that in GC_malloc

If the size argument is zero, then it is treated as one.

* include/gc/gc.h (GC_malloc_many): Move comment from mallocx.c;
document the case of zero value of the argument.
* mallocx.c (GC_malloc_many): If lb is zero, then set it to 1 (before
computing lg).

include/gc/gc.h

@@ -1951,9 +1951,12 @@ GC_API void GC_CALL GC_debug_ptr_store_and_dirty(void * /* p */,
 /* This returns a list of objects, linked through their first word.     */
 /* Its use can greatly reduce lock contention problems, since the       */
 /* allocator lock can be acquired and released many fewer times.        */
+/* Note that there is no "atomic" version of this function, as          */
+/* otherwise the links would not be seen by the collector.              */
+/* If the argument is zero, then it is treated as one.                  */
 GC_API GC_ATTR_MALLOC void * GC_CALL GC_malloc_many(size_t /* lb */);
 #define GC_NEXT(p) (*(void * *)(p))     /* Retrieve the next element    */
-                                        /* in returned list.            */
+                                        /* in the returned list.        */
 
 /* A filter function to control the scanning of dynamic libraries.      */
 /* If implemented, called by GC before registering a dynamic library    */

mallocx.c

@@ -444,13 +444,13 @@ GC_API void GC_CALL GC_generic_malloc_many(size_t lb, int k, void **result)
     (void) GC_clear_stack(0);
 }
 
-/* Note that the "atomic" version of this would be unsafe, since the    */
-/* links would not be seen by the collector.                            */
 GC_API GC_ATTR_MALLOC void * GC_CALL GC_malloc_many(size_t lb)
 {
     void *result;
-    size_t lg = ALLOC_REQUEST_GRANS(lb);
+    size_t lg;
 
+    if (EXPECT(0 == lb, FALSE)) lb = 1;
+    lg = ALLOC_REQUEST_GRANS(lb);
     GC_generic_malloc_many(GRANULES_TO_BYTES(lg), NORMAL, &result);
     return result;
 }