[libc] calloc must check for overflow (C99)

I think this was first standardized in C99, but was commonly checked for
earlier than that.

elkscmd/test/libc/malloc.c

@@ -53,7 +53,9 @@ TEST_CASE(malloc_calloc) {
         free(p);
     }
 
-    /* TODO check for mult overflow */
+    p = calloc(((unsigned)-1)>>2, 5);
+    EXPECT_EQ(errno, ENOMEM);
+    EXPECT_EQ_P(p, NULL);
 }
 
 TEST_CASE(malloc_realloc) {

libc/malloc/calloc.c

@@ -1,13 +1,27 @@
+#include <errno.h>
 #include <malloc.h>
 #include <string.h>
 
 void *
 calloc(unsigned int elm, unsigned int sz)
 {
-    register unsigned int v;
-    register void *ptr;
+    unsigned int v;
+    void *ptr;
 
-    ptr = malloc(v = elm * sz);
+#ifdef __GNUC__
+    if (__builtin_umul_overflow(elm, sz, &v)) {
+        errno = ENOMEM;
+        return 0;
+    }
+#else
+    v = elm * sz;
+    if (sz != 0 && v / sz != elm) {
+        errno = ENOMEM;
+        return 0;
+    }
+#endif
+
+    ptr = malloc(v);
     if (ptr)
         memset(ptr, 0, v);