fix compiling gcc 6.3.0 with strict c++ compilers

this patch is a backport of gcc revision 239971 to gcc 6.3.0.

submitted by osvein as PR#35.

patches/gcc-6.3.0/0014-ubsan-empty-string-fix.diff

@@ -0,0 +1,13 @@
+diff --git a/gcc/ubsan.c b/gcc/ubsan.c
+index 56637d8..1093824 100644
+--- a/gcc/ubsan.c
++++ b/gcc/ubsan.c
+@@ -1471,7 +1471,7 @@ ubsan_use_new_style_p (location_t loc)
+ 
+   expanded_location xloc = expand_location (loc);
+   if (xloc.file == NULL || strncmp (xloc.file, "\1", 2) == 0
+-      || xloc.file == '\0' || xloc.file[0] == '\xff'
++      || xloc.file[0] == '\0' || xloc.file[0] == '\xff'
+       || xloc.file[1] == '\xff')
+     return false;
+