[GR-19326] Fix potential NPE in REnvVars

(cherry picked from commit 9c3ebe6)

Author: steve-s

com.oracle.truffle.r.nodes/src/com/oracle/truffle/r/nodes/access/vector/CachedExtractVectorNode.java

@@ -308,6 +308,8 @@ private void applyDimensions(RAbstractContainer originalTarget, RAbstractVector
                     }
                     newDimNames[dimIndex] = result;
                     if (newDimNamesNames != null) {
+                        // (newDimNamesNames == null) <=> (originalDimNamesNames == null)
+                        assert originalDimNamesNames != null;
                         newDimNamesNames[dimIndex] = originalDimNamesNames.getDataAt(i);
                     }
                 }

com.oracle.truffle.r.runtime/src/com/oracle/truffle/r/runtime/REnvVars.java

@@ -126,7 +126,7 @@ public RContext.ContextState initialize(RContext context) {
                 }
             }
             TruffleFile userFile = userFilePath != null ? FileSystemUtils.getSafeTruffleFile(env, userFilePath) : null;
-            if (userFile.exists()) {
+            if (userFile != null && userFile.exists()) {
                 safeReadEnvironFile(userFile);
             }
         }