Fix additional warnings

When compiling with a newer version of glibc (2.35), some more warnings are being caught.

• Two of these warnings involve lines which create copies with auto, which can be fixed by using references.
• The other warning is -Wfloat-equal when comparing the return value of strtod to HUGE_VAL, which is a special value that is explicitly returned, so it should be okay to make the exact comparison. This warning can either be ignored or the code can be changed to check whether the last error was ERANGE.