ld2string should fail if string contains \0 in the middle

This bug affected RM_StringToLongDouble and HINCRBYFLOAT.
I added tests for both cases.

Main changes:
1. Fixed string2ld to fail if string contains \0 in the middle
2. Use string2ld in getLongDoubleFromObject - No point of
   having duplicated code here

The two changes above broke RM_SaveLongDouble/RM_LoadLongDouble
because the long double string was saved with length+1 (An innocent
mistake, but it's actually a bug - The length passed to
RM_SaveLongDouble should not include the last \0).

src/util.c

@@ -471,13 +471,14 @@ int string2ld(const char *s, size_t slen, long double *dp) {
     long double value;
     char *eptr;
 
-    if (slen >= sizeof(buf)) return 0;
+    if (slen == 0 || slen >= sizeof(buf)) return 0;
     memcpy(buf,s,slen);
     buf[slen] = '\0';
 
     errno = 0;
     value = strtold(buf, &eptr);
     if (isspace(buf[0]) || eptr[0] != '\0' ||
+        (size_t)(eptr-buf) != slen ||
         (errno == ERANGE &&
             (value == HUGE_VAL || value == -HUGE_VAL || value == 0)) ||
         errno == EINVAL ||