Fix calloc_explicitly_typed in case of lb*n overflow

* typd_mlc.c: Include limits.h (for SIZE_MAX).
* typd_mlc.c (GC_SIZE_MAX, GC_SQRT_SIZE_MAX): New macro (same as in
malloc.c).
* typd_mlc.c (GC_calloc_explicitly_typed): Return NULL if lb * n
overflows (same algorithm as in calloc defined in malloc.c); eliminate
lb *= n code duplication.

typd_mlc.c

@@ -627,6 +627,15 @@ GC_API GC_ATTR_MALLOC void * GC_CALL
    return((void *) op);
 }
 
+#include <limits.h>
+#ifdef SIZE_MAX
+# define GC_SIZE_MAX SIZE_MAX
+#else
+# define GC_SIZE_MAX (~(size_t)0)
+#endif
+
+#define GC_SQRT_SIZE_MAX ((1U << (WORDSZ / 2)) - 1)
+
 GC_API GC_ATTR_MALLOC void * GC_CALL GC_calloc_explicitly_typed(size_t n,
                                                         size_t lb, GC_descr d)
 {
@@ -640,15 +649,18 @@ GC_API GC_ATTR_MALLOC void * GC_CALL GC_calloc_explicitly_typed(size_t n,
     GC_ASSERT(GC_explicit_typing_initialized);
     descr_type = GC_make_array_descriptor((word)n, (word)lb, d, &simple_descr,
                                           &complex_descr, &leaf);
+    if ((lb | n) > GC_SQRT_SIZE_MAX /* fast initial check */
+        && lb > 0 && n > GC_SIZE_MAX / lb)
+      return NULL; /* n*lb overflow */
+    lb *= n;
     switch(descr_type) {
         case NO_MEM: return(0);
-        case SIMPLE: return(GC_malloc_explicitly_typed(n*lb, simple_descr));
+        case SIMPLE:
+            return GC_malloc_explicitly_typed(lb, simple_descr);
         case LEAF:
-            lb *= n;
             lb += sizeof(struct LeafDescriptor) + TYPD_EXTRA_BYTES;
             break;
         case COMPLEX:
-            lb *= n;
             lb += TYPD_EXTRA_BYTES;
             break;
     }