config: value_to_*(): don't overwrite result variable on error

Some of the value_to_*() functions wrote directly to the output
variable, even when the value was invalid. This often resulted in the
an actual configuration option (i.e. a member in the config struct) to
be overwritten by an invalid value.

For example, -o initial-color-theme=0 would set
conf->initial_color_theme to -1, resulting in a crash later, when
initializing a terminal instance.

CHANGELOG.md

@@ -70,7 +70,7 @@
 ### Changed
 
 * The label letters are no longer sorted before being assigned to URLs
-  ([#2140]2140[]).
+  ([#2140][2140]).
 * Sending SIGUSR1/SIGUSR2 to a `foot --server` process now causes
   newly spawned client instances to use the selected theme, instead of
   the original one.
@@ -83,6 +83,11 @@
 ### Deprecated
 ### Removed
 ### Fixed
+
+* Invalid configuration values overriding valid ones in surprising
+  ways.
+
+
 ### Security
 ### Contributors
 

config.c

@@ -474,8 +474,12 @@ str_to_ulong(const char *s, int base, unsigned long *res)
     errno = 0;
     char *end = NULL;
 
-    *res = strtoul(s, &end, base);
-    return errno == 0 && *end == '\0';
+    unsigned long v = strtoul(s, &end, base);
+    if (!(errno == 0 && *end == '\0'))
+        return false;
+
+    *res = v;
+    return true;
 }
 
 static bool NOINLINE
@@ -544,12 +548,13 @@ value_to_float(struct context *ctx, float *res)
     errno = 0;
     char *end = NULL;
 
-    *res = strtof(s, &end);
+    float v = strtof(s, &end);
     if (!(errno == 0 && *end == '\0')) {
         LOG_CONTEXTUAL_ERR("invalid decimal value");
         return false;
     }
 
+    *res = v;
     return true;
 }
 
@@ -641,7 +646,6 @@ value_to_enum(struct context *ctx, const char **value_map, int *res)
         valid_values[idx - 2] = '\0';
 
     LOG_CONTEXTUAL_ERR("not one of %s", valid_values);
-    *res = -1;
     return false;
 }
 
@@ -690,14 +694,18 @@ value_to_two_colors(struct context *ctx,
         goto out;
     }
 
+    uint32_t a, b;
+
     ctx->value = first_as_str;
-    if (!value_to_color(ctx, first, allow_alpha))
+    if (!value_to_color(ctx, &a, allow_alpha))
         goto out;
 
     ctx->value = second_as_str;
-    if (!value_to_color(ctx, second, allow_alpha))
+    if (!value_to_color(ctx, &b, allow_alpha))
         goto out;
 
+    *first = a;
+    *second = b;
     ret = true;
 
 out: