Reuse result from getenv

[mabraham]

Description

In theory, calling getenv a second time could return NULL after previously returning non-NULL, which would be undefined behavior when passed to strcmp. Re-using the non-NULL value is slightly safer, and keeps clang's static analyzer happier.

Target release

I would like my code to appear in release XXXXX

Type of contribution

• changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• changes to a module not authored by you
• new module contribution or edit of a module authored by you

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

• the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.

[mabraham]

Found during GROMACS CI version upgrade. Upstreaming the change for future convenience.

[GiovanniBussi]

I see, this would happen if:

• getenv is called
• context switch to another thread
• the other thread changes the environment variable
• context switch to the initial thread
• getenv is called again

Unlikely, but I agree should be fixed, thanks!