Protect against undefined behavior from inputs like "NaN"

Reproducer was (against commit 07ce2f9 on master):
> # make clean all {C,LD}FLAGS='-fsanitize=undefined -fno-sanitize-recover=all' \
>     && ./ttyplot <<<NaN |& tee /tmp/foo \
>     && reset \
>     && grep -oE ttyplot\.c.+ /tmp/foo

With GCC you would get:
> ttyplot.c:129:9: runtime error: signed integer overflow: 0 - -2147483648 cannot be represented in type 'int'

With Clang you would get:
> ttyplot.c:144:58: runtime error: nan is outside the range of representable values of type 'int'

ttyplot.c

@@ -9,6 +9,7 @@
 
 #include <assert.h>
 #include <ctype.h>  // isspace
+#include <math.h>
 #include <stdbool.h>
 #include <stdio.h>
 #include <string.h>
@@ -529,6 +530,11 @@ int main(int argc, char *argv[]) {
         else
             n=0;
 
+        if (! isfinite(d1))
+            d1 = 0.0;
+        if (two && ! isfinite(d2))
+            d2 = 0.0;
+
         values1[n] = d1;
         values2[n] = d2;