Cygwin: spawnvp, spawnvpe: fail if executable is not in $PATH

Call find_exec with the FE_NNF flag to enforce a NULL return when the
executable isn't found in $PATH.  Convert NULL to "".  This aligns
spawnvp and spawnvpe with execvp and execvpe.

winsup/cygwin/release/3.1.0

@@ -91,3 +91,6 @@ Bug Fixes
 - If the argument to mkdir(2) or rmdir(2) is 'x:\', don't strip the
   trailing backslash.
   Addresses: https://cygwin.com/ml/cygwin/2019-08/msg00334.html
+
+- Make spawnvp, spawnvpe fail if the executable is not in $PATH.
+  Addresses: https://cygwin.com/ml/cygwin/2019-10/msg00032.html

winsup/cygwin/spawn.cc

@@ -1081,8 +1081,9 @@ extern "C" int
 spawnvp (int mode, const char *file, const char * const *argv)
 {
   path_conv buf;
-  return spawnve (mode | _P_PATH_TYPE_EXEC, find_exec (file, buf), argv,
-		  cur_environ ());
+  return spawnve (mode | _P_PATH_TYPE_EXEC,
+		  find_exec (file, buf, "PATH", FE_NNF) ?: "",
+		  argv, cur_environ ());
 }
 
 extern "C" int
@@ -1090,7 +1091,9 @@ spawnvpe (int mode, const char *file, const char * const *argv,
 	  const char * const *envp)
 {
   path_conv buf;
-  return spawnve (mode | _P_PATH_TYPE_EXEC, find_exec (file, buf), argv, envp);
+  return spawnve (mode | _P_PATH_TYPE_EXEC,
+		  find_exec (file, buf, "PATH", FE_NNF) ?: "",
+		  argv, envp);
 }
 
 int