add: length check guard before sign check

Author: Mahmood-Sinan

src/strings/stdlib_str2num.fypp

@@ -138,9 +138,11 @@ module stdlib_str2num
         !----------------------------------------------
         ! Verify leading negative
         sign = 1
-        if( iachar(s(p:p)) == minus_sign+digit_0 ) then
-            sign = -1
-            p = p + 1
+        if(p<=len(s)) then
+            if(iachar(s(p:p)) == minus_sign+digit_0 ) then
+                sign = -1
+                p = p + 1
+            end if
         end if
         v = 0
         !----------------------------------------------