Fixed MD5 stack overflow bug on macOS

Specifically, the std::string(const char* s, size_t n) constructor on macOS seems to ignore the n parameter and overflows. This was mitigated by adding a terminating character to s and using the std::string(const char* s) constructor instead.

Author: alancleary

libtiledbvcf/external/md5/md5.cc

@@ -206,9 +206,10 @@ void md5_to_hex(const uint8_t* input, char* output) {
 }
 
 std::string md5_to_hex(const uint8_t* input) {
-  char output[32];
+  char output[33];
   md5_to_hex(input, output);
-  return std::string(output, 32);
+  output[32] = '\0';
+  return std::string(output);
 }
 
 }  // namespace md5