fix undefined behavior: left shift a negative integer

C/C++ leaves left-shifting a negative integer up to the compiler
implementation.

On most platforms, left-shifting a negative integer is the same as
casting to the unsigned equivalent, then casting back into the signed
type.

This replaces the left-shift with the mathematical equivalent and avoids
the undefined behavior.

Ideally this should be performed for any case of left-shifting a signed
type, but this commit only covers cases picked up by the undefined
behavior sanitizer.

src/lib/specGapFilling.cpp

@@ -268,7 +268,7 @@ uint8_t SpecGapFiller::getSpecGapFillParams (const SfbQuantizer& sfbQuantizer, c
       {
         unsigned countOld = 0, countNew = 0;
 
-        b = CLIP_PM (((b << 8) + (a >> 1)) / a, SHRT_MAX);
+        b = CLIP_PM (((b * (1 << 8)) + (a >> 1)) / a, SHRT_MAX);
         a = ((ySum << 8) - b * xSum + (size >> 1)) / size;
 
         ySum = grpScFacs[start];