Don't try to turn Argdata into string command line arguments.

Rust will always make use of the main() function on startup, even if we
make use of Argdata. Optimize this scenario a bit by checking whether
the provided Argdata is binary. If so, don't try to convert it to string
command line arguments.

src/libc/program/program_main.c

@@ -14,14 +14,22 @@ int main(int, char **, char **);
 // Fallback for when the program does not contain program_main(), so
 // that it can call into the C standard main() function.
 noreturn void program_main(const argdata_t *ad) {
+  // In case input starts with a low-value byte, it is very likely that
+  // the provided input is structured Argdata. Don't try to parse this
+  // input as string command line arguments.
+  size_t argdatalen =
+      __at_argdatalen > 0 && ((const uint8_t *)__at_argdata)[0] >= ' '
+          ? __at_argdatalen
+          : 0;
+
   // Copy the argument data over to a writable null-terminated buffer.
-  char arg[__at_argdatalen + 1];
-  memcpy(arg, __at_argdata, __at_argdatalen);
-  arg[__at_argdatalen] = '\0';
+  char arg[argdatalen + 1];
+  memcpy(arg, __at_argdata, argdatalen);
+  arg[argdatalen] = '\0';
 
   // Count the number of arguments.
   size_t argc = 0;
-  for (size_t i = 0; i <= __at_argdatalen; ++i)
+  for (size_t i = 0; i <= argdatalen; ++i)
     if (arg[i] == '\0')
       ++argc;