Parse subnormal numbers correctly

Fixes #202.

src/strconv.c

@@ -2,6 +2,7 @@
 #include <errno.h>
 #include <stdio.h>
 #include <string.h>
+#include <math.h>
 #include "jansson_private.h"
 #include "strbuffer.h"
 
@@ -69,7 +70,7 @@ int jsonp_strtod(strbuffer_t *strbuffer, double *out)
     value = strtod(strbuffer->value, &end);
     assert(end == strbuffer->value + strbuffer->length);
 
-    if(errno == ERANGE && value != 0) {
+    if((value == HUGE_VAL || value == -HUGE_VAL) && errno == ERANGE) {
         /* Overflow */
         return -1;
     }

test/suites/valid/real-subnormal-number/input

@@ -0,0 +1 @@
+[1.8011670033376514e-308]

test/suites/valid/real-subnormal-number/output

@@ -0,0 +1 @@
+[1.8011670033376514e-308]