CSS rgb(a) parsing: avoid uninitialized-value

We check that the input has at least 5 characters before reading
characters[4].

An rgb(a) color contains at least 3 numbers or percentages,
plus separator characters, so the length of a valid rgb(a) color
will be well over 5.

TBR=nainar@chromium.org
BUG=792306

Change-Id: Iceb49992ee7c5da346e2f64cbc7ccb82ad619f81
Reviewed-on: https://chromium-review.googlesource.com/811784Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522203}

third_party/WebKit/Source/core/css/parser/CSSParserFastPaths.cpp

@@ -426,7 +426,7 @@ static inline bool ParseAlphaValue(const CharacterType*& string,
 template <typename CharacterType>
 static inline bool MightBeRGBOrRGBA(const CharacterType* characters,
                                     unsigned length) {
-  if (length < 4)
+  if (length < 5)
     return false;
   return IsASCIIAlphaCaselessEqual(characters[0], 'r') &&
          IsASCIIAlphaCaselessEqual(characters[1], 'g') &&