Add unit tests for fix_negative

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index 962d5af..b167443 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -25,6 +25,8 @@
 #include "mbedtls/platform_util.h"
 #include "mbedtls/error.h"
 
+#include "ecp_invasive.h"
+
 #include <string.h>
 
 #if !defined(MBEDTLS_ECP_ALT)
@@ -1028,13 +1030,14 @@
     STORE32; i++;                               \
     cur = c > 0 ? c : 0; STORE32;               \
     cur = 0; while( ++i < MAX32 ) { STORE32; }  \
-    if( c < 0 ) fix_negative( N, c, bits );
+    if( c < 0 ) mbedtls_ecp_fix_negative( N, c, bits );
 
 /*
  * If the result is negative, we get it in the form
  * c * 2^(bits + 32) + N, with c negative and N positive shorter than 'bits'
  */
-static inline void fix_negative( mbedtls_mpi *N, signed char c, size_t bits )
+MBEDTLS_STATIC_TESTABLE
+void mbedtls_ecp_fix_negative( mbedtls_mpi *N, signed char c, size_t bits )
 {
     size_t i;