Prevent LookupDict2 from overflowing wtab

LookupDict2 looks forward in the wtab array, it should still stop at its
end. Otherwise the memory sanitizer reports this:

testing en A. B C, D. E: F.
==65960==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x7ff9d7ef0de8 in LookupDict2 /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/dictionary.c:2676:11
    #1 0x7ff9d7eec2ec in LookupDictList /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/dictionary.c:2899:10
    #2 0x7ff9d802860a in TranslateWord3 /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/translate.c:588:12
    #3 0x7ff9d80249d4 in TranslateWord /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/translate.c:1100:14
    #4 0x7ff9d8051fe0 in TranslateWord2 /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/translate.c:1361:11
    #5 0x7ff9d804885c in TranslateClause /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/translate.c:2623:17
    #6 0x7ff9d800e4e9 in SpeakNextClause /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/synthesize.c:1569:2
    #7 0x7ff9d7fa50e6 in Synthesize /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/speech.c:457:2
    #8 0x7ff9d7fa41b3 in sync_espeak_Synth /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/speech.c:570:29
    #9 0x7ff9d7fa872f in espeak_ng_Synthesize /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/speech.c:678:10
    #10 0x7ff9d7f06584 in espeak_Synth /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/espeak_api.c:90:32
    #11 0x4a8be3 in main /home/samy/brl/speech/espeak-ng-git/src/espeak-ng.c:691:3
    #12 0x7ff9d78297fc in __libc_start_main csu/../csu/libc-start.c:332:16
    #13 0x421449 in _start (/home/samy/ens/projet/1/speech/espeak-ng-git/src/.libs/espeak-ng+0x421449)

  Uninitialized value was created by an allocation of 'words' in the stack frame of function 'TranslateClause'
    #0 0x7ff9d8035380 in TranslateClause /home/samy/brl/speech/espeak-ng-git/src/libespeak-ng/translate.c:1941

src/libespeak-ng/dictionary.c

 				// don't use the contraction if any of the words are emphasized
 				//  or has an embedded command, such as MARK
 				if (wtab != NULL) {
-					for (ix = 0; ix <= skipwords; ix++) {
+					for (ix = 0; ix <= skipwords && wtab[ix].length; ix++) {
 						if (wtab[ix].flags & FLAG_EMPHASIZED2)
 							condition_failed = 1;
 					}