Prevent attempts to `nvm use` a circular reference.

nvm.sh

@@ -821,6 +821,9 @@ nvm() {
           echo "System version of node not found." >&2
           return 127
         fi
+      elif [ "_$VERSION" = "_∞" ]; then
+        echo "The alias \"$2\" leads to an infinite loop. Aborting." >&2
+        return 8
       fi
 
       local NVM_VERSION_DIR

test/fast/Running "nvm use foo" where "foo" is circular aborts

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+die () { echo $@ ; cleanup ; exit 1; }
+
+cleanup() {
+  rm -rf ../../alias/foo
+}
+
+. ../../nvm.sh
+
+echo 'foo' > ../../alias/foo
+
+OUTPUT="$(nvm use foo 2>&1)"
+EXPECTED_OUTPUT='The alias "foo" leads to an infinite loop. Aborting.'
+[ "_$OUTPUT" = "_$EXPECTED_OUTPUT" ] \
+  || die "'nvm use foo' did not output '$EXPECTED_OUTPUT'; got '$OUTPUT'"
+
+EXIT_CODE="$(nvm use foo 2>/dev/null ; echo $?)"
+[ "_$EXIT_CODE" = "_8" ] || die "Expected exit code 8; got $EXIT_CODE"
+
+cleanup;
+