cutils: refine strtol error handling in parse_debug_env

Avoid truncation of a 64-bit long to a 32-bit int, and check for errno
(especially ERANGE).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

util/cutils.c

@@ -537,16 +537,17 @@ int parse_debug_env(const char *name, int max, int initial)
 {
     char *debug_env = getenv(name);
     char *inv = NULL;
-    int debug;
+    long debug;
 
     if (!debug_env) {
         return initial;
     }
+    errno = 0;
     debug = strtol(debug_env, &inv, 10);
     if (inv == debug_env) {
         return initial;
     }
-    if (debug < 0 || debug > max) {
+    if (debug < 0 || debug > max || errno != 0) {
         fprintf(stderr, "warning: %s not in [0, %d]", name, max);
         return initial;
     }