Fixed integer-overflow in ToInt32().

error: negation of -2147483648 cannot be represented in type 'int32_t'.

Found by OSS-Fuzz.
diff --git a/src/njs_number.h b/src/njs_number.h
index b0f50dc..53800cd 100644
--- a/src/njs_number.h
+++ b/src/njs_number.h
@@ -76,7 +76,7 @@
 njs_inline int32_t
 njs_number_to_int32(double num)
 {
-    int32_t           r;
+    uint32_t          r;
     uint64_t          v;
     njs_int_t         exp;
     njs_diyfp_conv_t  conv;