Check the right element in isBook

This makes sure we read the right array element, corresponding
to all other array accesses in the same function, and all other
use of the isBook array elsewhere in the library.

This makes the fix in the previous commit ("Avoid too big changes
in isScale") work as intended, fixing other samples that triggered
assertions.

libAACenc/src/line_pe.cpp

@@ -184,7 +184,7 @@ void FDKaacEnc_calcSfbPe(PE_CHANNEL_DATA  *RESTRICT peChanData,
          }
          peChanData->sfbNActiveLines[sfbGrp+sfb] = nLines;
       }
-      else if( isBook[sfb] ) {
+      else if( isBook[sfbGrp+sfb] ) {
         /* provide for cost of scale factor for Intensity */
         INT delta = isScale[sfbGrp+sfb] - lastValIs;
         lastValIs = isScale[sfbGrp+sfb];