angle: fix potential nullptr dereference

`checkIsScalarBool` immediately deref's `typedCond`, which may be NULL.
Prevent that.

Caught by the static analyzer:
>
device/generic/vulkan-cereal/third-party/angle/src/compiler/translator/ParseContext.cpp:893:9:
warning: Called C++ object pointer is null
[clang-analyzer-core.CallAndMessage]

Bug: b/206470603
Test: None
Change-Id: I2c055d637fb3ef254c0cebfae32bfbe17bf850ea
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3935460
Auto-Submit: Greg Schlomoff <gregschlom@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>

src/compiler/translator/ParseContext.cpp

@@ -2671,7 +2671,7 @@ TIntermNode *TParseContext::addLoop(TLoopType type,
     }
     if (cond == nullptr || typedCond)
     {
-        if (type == ELoopDoWhile)
+        if (type == ELoopDoWhile && typedCond)
         {
             checkIsScalarBool(line, typedCond);
         }