Fix GCC7 warnings.

Taken from upstream:
https://git.savannah.gnu.org/cgit/aspell.git/commit/?id=8089fa02122fed0a6394eba14bbedcb1d18e2384

diff --git a/modules/filter/tex.cpp b/modules/filter/tex.cpp
index a979539..19ab63c 100644
--- a/modules/filter/tex.cpp
+++ b/modules/filter/tex.cpp
@@ -174,7 +174,7 @@ namespace {
 
     if (c == '{') {
 
-      if (top.in_what == Parm || top.in_what == Opt || top.do_check == '\0')
+      if (top.in_what == Parm || top.in_what == Opt || *top.do_check == '\0')
 	push_command(Parm);
 
       top.in_what = Parm;
diff --git a/prog/check_funs.cpp b/prog/check_funs.cpp
index db54f3d..89ee09d 100644
--- a/prog/check_funs.cpp
+++ b/prog/check_funs.cpp
@@ -647,7 +647,7 @@ static void print_truncate(FILE * out, const char * word, int width) {
     }
   }
   if (i == width-1) {
-    if (word == '\0')
+    if (*word == '\0')
       put(out,' ');
     else if (word[len] == '\0')
       put(out, word, len);