util: random_string is now more entropic (commaai#29312)

* random_string now returns a significantly higher entropy random string

sizeof(char*) is going to be 8 on a 64-bit system, not the length of the
string. Before, only strings of integer values [0-6] were being generated. Also
switched to C++ constructs for simplicity.

* Update common/util.cc

* maintain repo style

---------

Co-authored-by: Shane Smiskol <shane@smiskol.com>

common/util.cc

@@ -214,9 +214,9 @@ std::string hexdump(const uint8_t* in, const size_t size) {
 }
 
 std::string random_string(std::string::size_type length) {
-  const char* chrs = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+  const std::string chrs = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
   std::mt19937 rg{std::random_device{}()};
-  std::uniform_int_distribution<std::string::size_type> pick(0, sizeof(chrs) - 2);
+  std::uniform_int_distribution<std::string::size_type> pick(0, chrs.length() - 1);
   std::string s;
   s.reserve(length);
   while (length--) {