Fix array out of bounds bug in processing if-else-endif macros.

If we are not inside an if macro, we cannot simply decrease
elsetracker.

Fixes https://github.com/KhronosGroup/glslang/issues/29.

Test/baseResults/preprocessor.many.endif.vert.err

@@ -0,0 +1,12 @@
+ERROR: 0:1: '#endif' : mismatched statements 
+ERROR: 0:2: '#endif' : mismatched statements 
+ERROR: 0:3: '#endif' : mismatched statements 
+ERROR: 0:4: '#endif' : mismatched statements 
+ERROR: 0:5: '#endif' : mismatched statements 
+ERROR: 0:6: '#endif' : mismatched statements 
+ERROR: 0:7: '#endif' : mismatched statements 
+ERROR: 0:10: 'preprocessor evaluation' : bad expression 
+ERROR: 0:11: '' : missing #endif 
+ERROR: 9 compilation errors.  No code generated.
+
+

Test/preprocessor.many.endif.vert

@@ -0,0 +1,10 @@
+#endif
+#endif
+#endif
+#endif
+#endif
+#endif
+#endif
+
+#if
+#else

Test/test-preprocessor-list

@@ -12,3 +12,4 @@ preprocessor.pragma.vert
 preprocessor.simple.vert
 preprocessor.success_if_parse_would_fail.vert
 preprocessor.defined.vert
+preprocessor.many.endif.vert