[Fix] nvm_print_implicit_alias: keep STABLE bound, and STABLE and UNSTABLE independent of the caller

`STABLE` is only assigned inside the loop over `LAST_TWO`,
but it is echoed unconditionally afterwards.
With nothing installed, or with only an odd minor,
the loop never runs, so under `set -u` `nvm ls` dies on
`STABLE: unbound variable` instead of resolving `stable` to nothing.

`local STABLE` without an initializer also keeps the caller's value in sh and dash,
so `STABLE=x nvm ls stable` resolves the alias to something that isn't installed.
Initializing it where it is declared fixes both,
the way 139ed9c did for `PATTERN`, `NVM_LTS`, `LTS` and `VERSION`.
`local UNSTABLE` keeps the caller's value the same way,
which its `N/A` fallback doesn't cover, so it is initialized too.

The nounset test now covers the two states that leave `stable` unresolved,
and the caller test that `nvm ls stable` answers the same either way,
and that an outer `UNSTABLE` doesn't become the `unstable` alias.

Co-authored-by: NUAAwqy <146524024+NUAAwqy@users.noreply.github.com>
Co-authored-by: Jordan Harband <ljharb@gmail.com>

test/fast/Listing versions/Running 'nvm ls' with nounset should not fail

@@ -6,19 +6,29 @@ die () { echo "$@" ; exit 1; }
 \. ../../../nvm.sh
 \. ../../common.sh
 
-make_fake_node v0.12.34 || die 'fake v0.12.34 could not be made'
-
 # Enable no unset variable
 set -u
 
+# Nothing is installed, so there is no stable version to resolve to
+output=$(nvm ls 2>&1 1>/dev/null || true)
+test -z "${output}" || die "1: expected empty; got >${output}"
+
+make_fake_node v0.11.16 || die 'fake v0.11.16 could not be made'
+
+# Only an unstable version is installed, so `stable` still has nothing to resolve to
+output=$(nvm ls 2>&1 1>/dev/null || true)
+test -z "${output}" || die "2: expected empty; got >${output}"
+
+make_fake_node v0.12.34 || die 'fake v0.12.34 could not be made'
+
 # Try an alias that does not exist
 output=$(nvm ls 99 2>&1 1>/dev/null || true)
-test -z "${output}" || die "1: expected empty; got >${output}"
+test -z "${output}" || die "3: expected empty; got >${output}"
 
 # Try a version that does not exist
 output=$(nvm ls 0.12.00 2>&1 1>/dev/null || true)
-test -z "${output}" || die "2: expected empty; got >${output}"
+test -z "${output}" || die "4: expected empty; got >${output}"
 
 # Try a version that does exist
 output=$(nvm ls 0.12.34 2>&1 1>/dev/null || true)
-test -z "${output}" || die "3: expected empty; got >${output}"
+test -z "${output}" || die "5: expected empty; got >${output}"

test/fast/Unit tests/nvm commands ignore option variables set by the caller

@@ -54,4 +54,19 @@ VERSION=v20.0.0
 [ $? = 127 ] || die 'nvm use without a version or an .nvmrc should fail, rather than use an outer VERSION'
 unset VERSION
 
+# with nothing installed, `nvm ls stable` has no version to resolve to, so an outer STABLE must not supply one
+NVM_DIR="${TMP_DIR}/empty"
+mkdir -p "${NVM_DIR}"
+OUTPUT="$(nvm ls stable | tr -d ' \n')"
+STABLE=v20.0.0
+OUTPUT_WITH_STABLE="$(nvm ls stable | tr -d ' \n')"
+[ "_${OUTPUT_WITH_STABLE}" = "_${OUTPUT}" ] || die "nvm ls stable should ignore an outer STABLE; got >${OUTPUT_WITH_STABLE}< instead of >${OUTPUT}<"
+unset STABLE
+
+# likewise for an outer UNSTABLE; `nvm ls unstable` can't show it, since the leaked version isn't installed either
+UNSTABLE=v21.0.0
+OUTPUT="$(nvm_print_implicit_alias local unstable)"
+[ "_${OUTPUT}" = '_N/A' ] || die "nvm_print_implicit_alias local unstable should ignore an outer UNSTABLE; got >${OUTPUT}<"
+unset UNSTABLE
+
 cleanup