Fix overflow in delay (#40)

Gets checked in debug builds

CHANGELOG.md

@@ -12,6 +12,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - Support for STM32F091 - @jessebraham
 - Support for HSE as a system clocksource (#25 - breaking change) - @zklapow
 
+### Fixed
+
+- Fixed panic in delay overflow handling for debug builds - @david-sawatzke
+
 ## [v0.11.1] - 2019-01-05
 
 ### Added

src/delay.rs

@@ -99,7 +99,7 @@ impl DelayUs<u32> for Delay {
 
             let start_count = SYST::get_current();
             total_rvr -= current_rvr;
-            while ((start_count - SYST::get_current()) % MAX_SYSTICK) < current_rvr {}
+            while (start_count.wrapping_sub(SYST::get_current()) % MAX_SYSTICK) < current_rvr {}
         }
     }
 }