C library: make all branches of sqrt{,f,l} explicitly return

Makes GCC happier when doing library validation.

Our C front-end would implicitly generate nondet return values of the
correct type, so this does not actually change any verification
behaviour.

Author: tautschnig

src/ansi-c/library/math.c

@@ -758,7 +758,7 @@ float sqrtf(float f)
     case FE_TOWARDZERO :
       return (f - lowerSquare == 0.0f) ? lower : upper; break;
     default:;
-      //assert(0);
+      return __VERIFIER_nondet_float();
     }
 
   }
@@ -835,7 +835,7 @@ double sqrt(double d)
     case FE_TOWARDZERO:
       return (d - lowerSquare == 0.0) ? lower : upper; break;
     default:;
-      //assert(0);
+      return __VERIFIER_nondet_double();
     }
 
   }
@@ -906,7 +906,7 @@ long double sqrtl(long double d)
     case FE_TOWARDZERO:
       return (d - lowerSquare == 0.0l) ? lower : upper; break;
     default:;
-      //assert(0);
+      return __VERIFIER_nondet_long_double();
     }
 
   }