fix some bug of strtold

  with the test of sim:lua ,I have fixed some bug of error output
  1.input: "  " and error output string is "NULL"
  2.input: "1.2e+", error output string is "NULL"
  3.input: "."      error output string is "NULL"

Author: lilei19

libs/libc/stdlib/lib_strtold.c

@@ -119,7 +119,7 @@ static long_long scanexp(FAR char **f, bool flag)
 
   c = shgetc(s);
 
-  if (c == '+' || c == '-')
+  if ((c == '+' || c == '-') && isdigit(*s))
     {
       neg = (c == '-');
       c = shgetc(s);
@@ -335,13 +335,6 @@ static long_double decfloat(FAR char *ptr, FAR char **endptr)
         }
     }
 
-  if (num_digit == 0)
-    {
-      shunget(f);
-      ifexist(endptr, f);
-      return zero;
-    }
-
   if ((c | 32) == 'e')
     {
       num_decimal = scanexp(&f, 1) + num_decimal;
@@ -357,6 +350,11 @@ static long_double decfloat(FAR char *ptr, FAR char **endptr)
     }
 
   ifexist(endptr, f);
+  if (num_digit == 0)
+    {
+      return zero;
+    }
+
   f = ptr;
 
   k = 0;
@@ -475,7 +473,7 @@ static long_double hexfloat(FAR char *ptr,
         }
     }
 
-  for (; c - '0' < 10 || (c | 32) - 'a' < 6 || c == '.'; c = shgetc(f))
+  for (; isxdigit(c) || c == '.'; c = shgetc(f))
     {
       if (c == '.')
         {
@@ -519,7 +517,6 @@ static long_double hexfloat(FAR char *ptr,
 
   if (!gotdig)
     {
-      shunget(f);
       shunget(f);
       if (gotrad)
         {
@@ -642,7 +639,7 @@ static long_double hexfloat(FAR char *ptr,
 static long_double strtox(FAR const char *str, FAR char **endptr, int flag)
 {
   FAR char *s = (FAR char *)str;
-  int negative = 0;
+  bool negative = 0;
   long_double y = 0;
   int i = 0;
 
@@ -662,6 +659,7 @@ static long_double strtox(FAR const char *str, FAR char **endptr, int flag)
       case 3:
         bits = LDBL_MANT_DIG,
         emin = LDBL_MIN_EXP - bits;
+        break;
       default:
         return 0;
     }
@@ -718,10 +716,15 @@ static long_double strtox(FAR const char *str, FAR char **endptr, int flag)
       s += 2;
       y = hexfloat(s, endptr, bits, emin);
     }
-  else
+  else if (isdigit(*s) || (*s == '.' && isdigit(*(s + 1))))
     {
       y = decfloat(s, endptr);
     }
+  else
+    {
+      ifexist(endptr, (FAR char *)str);
+      return 0;
+    }
 
   return negative ? -y : y;
 }