Fix missing height check found by /analyze

A recent run of the experimental VC++ /analyze builder gave this
warning:

skia\src\gpu\gl\grglgpu.cpp(851) : warning C6287: Redundant code:  the left and right sub-expressions are identical.

The code it complained about was this:

    if (width < 0 || width < 0) {

which was introduced in https://skia-review.googlesource.com/c/20445/

This change makes the obvious fix.

Change-Id: I9d1743293fac9dd15ed82cf85efef907d727921e
Reviewed-on: https://skia-review.googlesource.com/21620
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Reviewed-by: Bruce Dawson <brucedawson@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>

src/gpu/gl/GrGLGpu.cpp

@@ -848,7 +848,7 @@ bool GrGLGpu::onTransferPixels(GrTexture* texture,
         rowBytes = trimRowBytes;
     }
     const void* pixels = (void*)offset;
-    if (width < 0 || width < 0) {
+    if (width < 0 || height < 0) {
         return false;
     }