Fix a problem formatting very large numbers. (#642)

Avoid walking off the end of an array when trying to format a number larger than 1000T.

Motivated by #641, as reported by @shingchuang, but slightly
reimplemented.

src/t_units.c

@@ -99,5 +99,12 @@ main(int argc, char **argv)
     unit_snprintf(s, 11, d, 'a');
     assert(strncmp(s, "35.2 Tbit", 11) == 0);
 
+    d = 4.0 * 1024 * 1024 * 1024 * 1024 * 1024;
+    unit_snprintf(s, 11, d, 'A');
+    assert(strncmp(s, "4096 TByte", 11) == 0);
+
+    unit_snprintf(s, 11, d, 'a');
+    assert(strncmp(s, "36029 Tbit", 11) == 0);
+
     return 0;
 }

src/units.c

@@ -303,14 +303,14 @@ extern    "C"
 
 		if (isupper((int) inFormat))
 		{
-		    while (tmpNum >= 1024.0 && conv <= TERA_CONV)
+		    while (tmpNum >= 1024.0 && conv < TERA_CONV)
 		    {
 			tmpNum /= 1024.0;
 			conv++;
 		    }
 		} else
 		{
-		    while (tmpNum >= 1000.0 && conv <= TERA_CONV)
+		    while (tmpNum >= 1000.0 && conv < TERA_CONV)
 		    {
 			tmpNum /= 1000.0;
 			conv++;