swr/rast: do not crash on NULL strings returned by getenv

swr/rast: do not crash on NULL strings returned by getenv

The current convenience function GetEnv feeds the results of getenv
directly into std::string(). That is a bad idea, since the variable
may be unset, thus we feed NULL into the C++ construct.

The latter of which is not allowed and leads to a crash.

v2: Better variable name, implicit char* -> std::string conversion (Eric)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101832
Fixes: a25093de718 ("swr/rast: Implement JIT shader caching to disk")
Cc: Tim Rowley <timothy.o.rowley@intel.com>
Cc: Laurent Carlier <lordheavym@gmail.com>
Cc: Bernhard Rosenkraenzer <bero@lindev.ch>
[Emil Velikov: make an actual commit from the misc diff]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com> (v1)
Reviewed-by: Laurent Carlier <lordheavym@gmail.com> (v1)