py/formatfloat: Fix case where floats could render with negative digits.

Prior to this patch, some architectures (eg unix x86) could render floats
with "negative" digits, like ")".  For example, '%.23e' % 1e-80 would come
out as "1.0000000000000000/)/(,*0e-80".  This patch fixes the known cases.

py/formatfloat.c

@@ -330,7 +330,11 @@ int mp_format_float(FPTYPE f, char *buf, size_t buf_size, char fmt, int prec, ch
     // Print the digits of the mantissa
     for (int i = 0; i < num_digits; ++i, --dec) {
         int32_t d = (int32_t)f;
-        *s++ = '0' + d;
+        if (d < 0) {
+            *s++ = '0';
+        } else {
+            *s++ = '0' + d;
+        }
         if (dec == 0 && prec > 0) {
             *s++ = '.';
         }

tests/float/float_format.py

@@ -13,3 +13,7 @@ for prec in range(8):
 # check certain cases that had a digit value of 10 render as a ":" character
 print('%.2e' % float('9' * 51 + 'e-39'))
 print('%.2e' % float('9' * 40 + 'e-21'))
+
+# check a case that would render negative digit values, eg ")" characters
+# the string is converted back to a float to check for no illegal characters
+float('%.23e' % 1e-80)