Fix null pointer dereference in redeclaration error message

When a function parameter name conflicts with another, the pointer
returned to ParseContext will be null.

BUG=chromium:745242
TEST=angle_unittests

Change-Id: Ie53bb06b0c6660e382d85aeda41f3a1b7df5a917
Reviewed-on: https://chromium-review.googlesource.com/603368
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
diff --git a/src/compiler/translator/ParseContext.cpp b/src/compiler/translator/ParseContext.cpp
index 7fa3b0e..a4796e3 100644
--- a/src/compiler/translator/ParseContext.cpp
+++ b/src/compiler/translator/ParseContext.cpp
@@ -3005,7 +3005,7 @@
                 }
                 else
                 {
-                    error(location, "redefinition", variable->getName().c_str());
+                    error(location, "redefinition", param.name->c_str());
                 }
             }
         }